Module should be decoupled from JSON requests/responses

[samnotsowise]

JSON is requests and responses are woven throughout this module. I suggest that requests should be interrogated (the Accept header) to determine the desired request handling/response format. If the module is unable to handle the format, a 406 Not Acceptable response should be given.

[acuthbert]

It kind of (mostly) is already - see https://github.com/RhubarbPHP/Module.RestApi/blob/master/src/UrlHandlers/RestResourceHandler.php
getJson can be complimented by getXml, getHtml etc.