Use _language field instead of language

 When we want to force a language instead of using Tika language detection, we set `language` field in documents.

 To be consistent with other forced fields, `_content_type` and `_name`, we should prefix `language` field by an underscore `_`.

 So `language` become `_language`.

 We first deprecate `language` in version 2.1.0 and we remove it in 2.3.0.

 Closes elastic#68.

(cherry picked from commit 2f46343)

README.md

@@ -46,13 +46,14 @@ In this case, the JSON to index can be:
 }
 ```
 
-Or it is possible to use more elaborated JSON if content type or resource name need to be set explicitly:
+Or it is possible to use more elaborated JSON if content type, resource name or language need to be set explicitly:
 
 ```javascript
 {
     "my_attachment" : {
         "_content_type" : "application/pdf",
         "_name" : "resource/name/of/my.pdf",
+        "_language" : "en",
         "content" : "... base64 encoded attachment ..."
     }
 }
@@ -121,7 +122,16 @@ By default, language detection is disabled (`false`) as it could come with a cos
 This default value can be changed by setting the `index.mapping.attachment.detect_language` setting.
 It can also be provided on a per document indexed using the `_detect_language` parameter.
 
-Note, this feature is supported since `2.0.0` version.
+Note that you can force language using `_language` field when sending your actual document:
+
+```javascript
+{
+    "my_attachment" : {
+        "_language" : "en",
+        "content" : "... base64 encoded attachment ..."
+    }
+}
+```
 
 Highlighting attachments
 ------------------------

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

@@ -352,8 +352,11 @@ public void parse(ParseContext context) throws IOException {
                     } else if ("_name".equals(currentFieldName)) {
                         name = parser.text();
                     } else if ("language".equals(currentFieldName)) {
-                        // TODO should be _language
+                        // TODO deprecated form. Will be removed in 2.3
                     	language = parser.text();
+                        logger.debug("`language` is now deprecated. Use `_language`. See https://github.com/elasticsearch/elasticsearch-mapper-attachments/issues/68");
+                    } else if ("_language".equals(currentFieldName)) {
+                        language = parser.text();
                     }
                 } else if (token == XContentParser.Token.VALUE_NUMBER) {
                     if ("_indexed_chars".equals(currentFieldName) || "_indexedChars".equals(currentFieldName)) {

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

@@ -74,7 +74,7 @@ private void testLanguage(String filename, String expected, String... forcedLang
                         .field("content", html);
 
         if (forcedLanguage.length > 0) {
-            xcb.field("language", forcedLanguage[0]);
+            xcb.field("_language", forcedLanguage[0]);
         }
 
         xcb.endObject().endObject();