Add toString() for FilterBuilders

Closes #2887.

src/main/java/org/elasticsearch/index/query/BaseFilterBuilder.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.index.query;
 
 import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentFactory;
 
 import java.io.IOException;
 
@@ -28,6 +29,18 @@ import java.io.IOException;
  */
 public abstract class BaseFilterBuilder implements FilterBuilder {
 
+    @Override
+    public String toString() {
+        try {
+            XContentBuilder builder = XContentFactory.jsonBuilder();
+            builder.prettyPrint();
+            toXContent(builder, EMPTY_PARAMS);
+            return builder.string();
+        } catch (Exception e) {
+            throw new QueryBuilderException("Failed to build filter", e);
+        }
+    }
+
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
         builder.startObject();