WP_JSON_Response -> WP_HTTP_Response

This is now a generic object that can be shared across the REST and HTTP
APIs.
Rename the class to a correct one
Rename ResponseInterface to match
Include both files correctly
Check correct interface in json_ensure_response
Use json_ensure_response instead of reinventing
Ensure ensure
Merge pull request #420 from WP-API/generic-http-response

Make HTTP response object generic