Add json/xml helpers

[barryvdh]

This adds a json and xml helper method to the HttpClient, to make it easier to request/process json or xml feeds.

• Json encodes the data to json before sending, and decodes the result to an array
• Xml sends the body as string, (encodes arrays as form data), and decodes the XML.

The Accept and Content-Type are set by default when not set.

Note that this slightly changes the ResponseParser::json() method to be inline with the normal json_decode options, but perhaps these can be made private, because these helpers would make them not so much needed.

[barryvdh]

Or should we decorate the response, instead of returning a plain psr-7 response?

[barryvdh]

See #163

[judgej]

Many gateways use different send and receive formats. I'm working on GiroCheckout where you send form-encoded and receive JSON on response. So the request and response data types are by no means paired up.

[barryvdh]

With the decorator we can decouple that, so we could have a method to send json/formdata, but process it separately.

[judgej]

Presumably a driver can extend that decorator to add other conversions too, such as CSV and name/value lines, just to keep the logic in the right place..

[barryvdh]

I think this would probably be a bit to opiniated.