GET requests are always using html format for the response.

Problem/Motivation

ContentNegotitation::getContentType() is not checking the format correctly. So basically if you try to do a GET request for a node, requesting for a hal_json format or json, you will receive html instead of the correct format.

Proposed resolution

Obtain the format from the request. I think the tests are working because we add _format directly

      // Read it over the REST API.
      $response = $this->httpRequest($entity->urlInfo()->setRouteParameter('_format', $this->defaultFormat), 'GET');
      $this->assertResponse('200', 'HTTP response code is correct.');
      $this->assertHeader('content-type', $this->defaultMimeType);
      $data = Json::decode($response);

I'm not sure why we are not obtaining the format... we are trying to obtain the format from the query and not sure if I'm missing something that has been changed and thats why we are doing it...

    if ($request->query->has('_format')) {
      return $request->query->get('_format');
    }