Minor changes to RestUtils.decodeQueryString() tests

Testing

    RestUtils.decodeQueryString("something", "something".indexOf('?') + 1, params);

is not really checking decoding of an empty query.
Instead, it is testing decoding of "something"
as a query (because "something.index('?')+1" evaluates to 0).
The parameter map is left empty because
"malformed" pairs like "something" are currently skipped.

Instead, this change modify this test to check the edge cases:

+ fromIndex >= queryString.length()
+ fromIndex < 0

modules/elasticsearch/src/test/java/org/elasticsearch/rest/util/RestUtilsTests.java

@@ -52,7 +52,12 @@ public class RestUtilsTests {
 
         params.clear();
         uri = "something";
-        RestUtils.decodeQueryString(uri, uri.indexOf('?') + 1, params);
+        RestUtils.decodeQueryString(uri, uri.length(), params);
+        assertThat(params.size(), equalTo(0));
+
+        params.clear();
+        uri = "something";
+        RestUtils.decodeQueryString(uri, -1, params);
         assertThat(params.size(), equalTo(0));
     }
 }