Added JSON payload support for POST

I was trying to send JSON inside the POST body when I realized qwest only seems to support form encoded data. I've made a little modification for it to support application/json content-type for requests. Main changes are, support for options.headers (as using the "before" functionality was not correct as you have to know request content-type prior to serializing the request body) and the adition of JSON.stringify in case of POST and */json.

Also I think there was some buggy code on lines 293 and 294 in case the header name comes already in the correct format.

Hope you find it useful.