Refactor/to x content fragments2 (#2329)

* Moves more classes over to ToXContentObject/Fragment

* Removes ToXContentToBytes

* Removes ToXContent from Enums

* review comment fix

* slight change to use XContantHelper

Author: colings86

server/src/main/java/org/elasticsearch/persistent/PersistentTaskParams.java

@@ -20,11 +20,11 @@
 package org.elasticsearch.persistent;
 
 import org.elasticsearch.common.io.stream.NamedWriteable;
-import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.ToXContentObject;
 
 /**
  * Parameters used to start persistent task
  */
-public interface PersistentTaskParams extends NamedWriteable, ToXContent {
+public interface PersistentTaskParams extends NamedWriteable, ToXContentObject {
 
 }