fixed [#1661] WAUrlFetch applies query string params and http headers after writing to response body resulting in an IllegalStateException

see [ticket #1661](http://play.lighthouseapp.com/projects/57987-play-framework/tickets/1661-waurlfetch-applies-query-string-params-and-http-headers-after-writing-to-response-body-resulting-in-an-illegalstateexception#ticket-1661-2)