add isClosed method to SensitiveXContentParser

Original commit: elastic/x-pack-elasticsearch@b1cada3bfc

watcher/src/main/java/org/elasticsearch/watcher/support/secret/SensitiveXContentParser.java

@@ -230,6 +230,11 @@ public class SensitiveXContentParser implements XContentParser {
         return parser.getTokenLocation();
     }
 
+    @Override
+    public boolean isClosed() {
+        return parser.isClosed();
+    }
+
     @Override
     public void close() throws ElasticsearchException {
         parser.close();