Add support for XML and JSON

[richmolj]

The beginning of support for multiple content types. You can now do this:

employees = EmployeeResource.all(params)
employees.to_json # simple plain json
employees.to_xml # plain json in xml format
employees.to_jsonapi # json in jsonapi format

All serialization, sparse fields and "sideloads" will be respected. When rendering JSON, the top-level keys are employees (or whatever the jsonapi "type" is) and meta. This way we can continue to support total count and other metadata.

With some minor additions to a Rails app + responders gem, which we will add to our generator, that means this:

def index
  employees = EmployeeResource.all(params)
  respond_with(employees)
end

Will render the approprate response based on the request format (.json, .xml, .jsonapi), or content type (application/json, application/xml, application/vnd.api+json').

Note there are no custom xml or json renderers registered in the Railtie! Our proxy object correctly responds to to_xml and to_json, so it works out-of-the-box, even alongside existing APIs already rendering vanilla json.

Some exceptions:

• Write payloads are still jsonapi-only. Would not be particularly hard to support other formats, but not worth even the minor effort at this point.
• This means we could accept different query payloads as well (again, just not worth it right now).
• Errors are still always in jsonapi format. Again, I'd like to have a real-life use case before supporting others.

Other notes:

• Ensure sideloads resolve with unique, non-nil parent ids by the new #ids_for_parents method, which will be passed to .scope or used within #load_params.