\Drupal\jsonapi\Controller\RequestHandler::deserializeBody() should not use the format corresponding to the Content-Type request header's value

#2934149: [>=8.5] JSON API routes not specifying _content_type_format route requirement, resulting in bad DX would've solved the root cause, and would've resulted in this not mattering, but there nevertheless is no point in making this dynamic when we can just hardcode it to api_json.

So this is apparently breaking an existing JSON API test. That test does:

    // 3. Missing Content-Type error.
    $response = $this->request('POST', $collection_url, [
      'body' => Json::encode($body),
      'auth' => [$this->user->getUsername(), $this->user->pass_raw],
      'headers' => ['Accept' => 'application/vnd.api+json'],
    ]);
    $created_response = Json::decode($response->getBody()->__toString());
    $this->assertEquals(422, $response->getStatusCode());
    $this->assertNotEmpty($created_response['errors']);
    $this->assertEquals('Unprocessable Entity', $created_response['errors'][0]['title']);

That looks as if it's testing that there's a helpful error response for a missing Content-Type request header. But the response looks like this:

 array(1) {
  ["errors"]=>
  array(1) {
    [0]=>
    array(5) {
      ["title"]=>
      string(20) "Unprocessable Entity"
      ["status"]=>
      int(422)
      ["detail"]=>
      string(101) "There was an error un-serializing the data. Message: Deserialization for the format  is not supported"
      ["links"]=>
      array(1) {
        ["info"]=>
        string(54) "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html"
      }
      ["code"]=>
      int(0)
    }
  }
}

So: Deserialization for the format is not supported — that's not remotely helpful.

The actually helpful error response would be added by #2934149: [>=8.5] JSON API routes not specifying _content_type_format route requirement, resulting in bad DX. I'm commenting this test coverage for now, in favor of #2934149: [>=8.5] JSON API routes not specifying _content_type_format route requirement, resulting in bad DX.

We won't forget about this, because A) there's a @todo, B) there's an issue that it points to: #2934149: [>=8.5] JSON API routes not specifying _content_type_format route requirement, resulting in bad DX.