Fix for jquery PUT, DELETE content type.

When sending a PUT or DELETE using Ajax, jquery sends a content type with charset=utf8 attached.  This causes piston to hiccup and not process the request.  This small patch fixes this.   I'm not sure if this is the correct place to put this, but I've had this in a piece of middleware for a while and was tired of distributing it through the app instead of in the Piston core.  Thanks.