Moves more classes over to ToXContentObject/Fragment (#2283)

Author: colings86

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

@@ -36,6 +36,7 @@
 import org.elasticsearch.common.xcontent.ObjectParser;
 import org.elasticsearch.common.xcontent.ObjectParser.NamedObjectParser;
 import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.ToXContentObject;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.tasks.Task;
@@ -266,7 +267,7 @@ public String toString() {
     /**
      * A record that represents a single running persistent task
      */
-    public static class PersistentTask<P extends PersistentTaskParams> implements Writeable, ToXContent {
+    public static class PersistentTask<P extends PersistentTaskParams> implements Writeable, ToXContentObject {
         private final String id;
         private final long allocationId;
         private final String taskName;