Deprecate `content` by `_content`

When we want to force some values, we need to set those using `_field` where `field` is the field name we want to force:

```
{
  "file": {
    "_name": "myfilename.txt"
  }
}
```

But to set the content itself, we use `content` field name.

```
{
  "file": {
    "content": "VGhpcyBpcyBhbiBlbGFzdGljc2VhcmNoIG1hcHBlciBhdHRhY2htZW50IHRlc3Qu",
    "_name": "myfilename.txt"
  }
}
```

For consistency, we set `_content` instead:

```
{
  "file": {
    "_content": "VGhpcyBpcyBhbiBlbGFzdGljc2VhcmNoIG1hcHBlciBhdHRhY2htZW50IHRlc3Qu",
    "_name": "myfilename.txt"
  }
}
```

Closes #73.

(cherry picked from commit 2e6be20)

README.md

@@ -54,7 +54,7 @@ Or it is possible to use more elaborated JSON if content type, resource name or
         "_content_type" : "application/pdf",
         "_name" : "resource/name/of/my.pdf",
         "_language" : "en",
-        "content" : "... base64 encoded attachment ..."
+        "_content" : "... base64 encoded attachment ..."
     }
 }
 ```
@@ -105,13 +105,11 @@ Indexed Characters
 
 By default, `100000` characters are extracted when indexing the content. This default value can be changed by setting the `index.mapping.attachment.indexed_chars` setting. It can also be provided on a per document indexed using the `_indexed_chars` parameter. `-1` can be set to extract all text, but note that all the text needs to be allowed to be represented in memory.
 
-Note, this feature is supported since `1.3.0` version.
-
 Metadata parsing error handling
 -------------------------------
 
 While extracting metadata content, errors could happen for example when parsing dates.
-Since version `1.9.0`, parsing errors are ignored so your document is indexed.
+Parsing errors are ignored so your document is indexed.
 
 You can disable this feature by setting the `index.mapping.attachment.ignore_errors` setting to `false`.
 
@@ -128,7 +126,7 @@ Note that you can force language using `_language` field when sending your actua
 {
     "my_attachment" : {
         "_language" : "en",
-        "content" : "... base64 encoded attachment ..."
+        "_content" : "... base64 encoded attachment ..."
     }
 }
 ```

src/main/java/org/elasticsearch/index/mapper/attachment/AttachmentMapper.java

@@ -345,7 +345,11 @@ public class AttachmentMapper implements Mapper {
                 if (token == XContentParser.Token.FIELD_NAME) {
                     currentFieldName = parser.currentName();
                 } else if (token == XContentParser.Token.VALUE_STRING) {
-                    if ("content".equals(currentFieldName)) {
+                    if ("_content".equals(currentFieldName)) {
+                        content = parser.binaryValue();
+                    } else if ("content".equals(currentFieldName)) {
+                        // TODO Remove in 2.4.0. See #75 https://github.com/elasticsearch/elasticsearch-mapper-attachments/issues/75
+                        logger.warn("`content` has been deprecated by _content. Please update your code. Will be removed in a future version.");
                         content = parser.binaryValue();
                     } else if ("_content_type".equals(currentFieldName)) {
                         contentType = parser.text();

src/test/java/org/elasticsearch/index/mapper/xcontent/LanguageDetectionAttachmentMapperTests.java

@@ -71,7 +71,7 @@ public class LanguageDetectionAttachmentMapperTests extends ElasticsearchTestCas
                     .field("_id", 1)
                     .startObject("file")
                         .field("_name", filename)
-                        .field("content", html);
+                        .field("_content", html);
 
         if (forcedLanguage.length > 0) {
             xcb.field("_language", forcedLanguage[0]);
@@ -128,7 +128,7 @@ public class LanguageDetectionAttachmentMapperTests extends ElasticsearchTestCas
                 .field("_id", 1)
                 .startObject("file")
                     .field("_name", "text-in-english.txt")
-                    .field("content", html)
+                    .field("_content", html)
                     .field("_detect_language", true)
                 .endObject().endObject();
 

src/test/java/org/elasticsearch/index/mapper/xcontent/MetadataMapperTest.java

@@ -57,7 +57,7 @@ public class MetadataMapperTest extends ElasticsearchTestCase {
                     .field("_id", 1)
                     .startObject("file")
                         .field("_name", filename)
-                        .field("content", html)
+                        .field("_content", html)
                     .endObject()
                 .endObject().bytes();
 

src/test/java/org/elasticsearch/plugin/mapper/attachments/test/SimpleAttachmentIntegrationTests.java

@@ -90,7 +90,7 @@ public class SimpleAttachmentIntegrationTests extends ElasticsearchIntegrationTe
 
         client().admin().indices().putMapping(putMappingRequest("test").type("person").source(mapping)).actionGet();
 
-        index("test", "person", jsonBuilder().startObject().field("file").startObject().field("content", txt).field("_indexed_chars", CONTENT_LENGTH_LIMIT).endObject());
+        index("test", "person", jsonBuilder().startObject().field("file").startObject().field("_content", txt).field("_indexed_chars", CONTENT_LENGTH_LIMIT).endObject());
         refresh();
 
         CountResponse countResponse = client().prepareCount("test").setQuery(queryString("BeforeLimit").defaultField("file")).execute().get();
@@ -108,7 +108,7 @@ public class SimpleAttachmentIntegrationTests extends ElasticsearchIntegrationTe
 
         client().admin().indices().putMapping(putMappingRequest("test").type("person").source(mapping)).actionGet();
 
-        index("test", "person", jsonBuilder().startObject().field("file").startObject().field("content", txt).field("_indexed_chars", CONTENT_LENGTH_LIMIT).endObject());
+        index("test", "person", jsonBuilder().startObject().field("file").startObject().field("_content", txt).field("_indexed_chars", CONTENT_LENGTH_LIMIT).endObject());
         refresh();
 
         CountResponse countResponse = client().prepareCount("test").setQuery(queryString("Begin").defaultField("file")).execute().get();
@@ -141,7 +141,7 @@ public class SimpleAttachmentIntegrationTests extends ElasticsearchIntegrationTe
 
         client().admin().indices().putMapping(putMappingRequest("test").type("person").source(mapping)).actionGet();
 
-        index("test", "person", jsonBuilder().startObject().field("file").startObject().field("content", txt)
+        index("test", "person", jsonBuilder().startObject().field("file").startObject().field("_content", txt)
                 .field("_content_type", dummyContentType)
                 .field("_name", dummyName)
                 .endObject());