The flushBuffer(http://docs.oracle.com/javaee/6/api/javax/servlet/ServletResponse.html#flushBuffer()) which causes the response to be marked as commited. and Adding afterwards any headers to the response has no effect.

We should override the that method and prevent it's default behaviour.

Also, it would be good idea to check the specs for any other method that could commit the message and prevent from enriching it by Warp.