Fix duplicate Content-Type request header in WS

Hi, 
WSClient adds Content-Type header even if user specify Content-Type already by using withHeaders. 
This behavior is different from Play2.2.x. See [here](https://github.com/playframework/playframework/blob/2.2.x/framework/src/play/src/main/scala/play/api/libs/ws/WS.scala#L558).