allow for json and xson to be parsed in content type

Author: kimchy

modules/elasticsearch/src/main/java/org/elasticsearch/util/xcontent/XContentType.java

@@ -39,11 +39,11 @@ public static XContentType fromRestContentType(String contentType) {
         if (contentType == null) {
             return null;
         }
-        if ("application/json".equals(contentType)) {
+        if ("application/json".equals(contentType) || "json".equalsIgnoreCase(contentType)) {
             return JSON;
         }
 
-        if ("application/xson".equals(contentType)) {
+        if ("application/xson".equals(contentType) || "xson".equalsIgnoreCase(contentType)) {
             return XSON;
         }