Sending url-encoded params, json or multipart requests with Guzzle is easy, because shortcuts are implemented within [`GuzzleHttp\Client::applyOptions()`](https://github.com/guzzle/guzzle/blob/7.5.0/src/Client.php#L340).

Currently, those shortcuts are unreachable through the League OAuth2 Client, and thus all its providers.

This fix relies on `AbstractProvider::getResponse()`, and does not affect `AbstractProvider::getAuthenticatedRequest()`.

`GuzzleHttp\ClientInterface::send()` method supports a second optional `$options` parameter [since v6.0](guzzle/guzzle@1a9ad6b?diff=split#diff-7fe9d7bab1528c58fb39365621c6f02430dd01828c26fe8b00f0408de72c71b8).

Upgrading `AbstractProvider::getResponse()` to support this second optional parameter allows the developper to use a `"form_params"`, `"json"` or `"multipart"` option key :
```php
$options = ['multipart' => $multipart];
$request = $oauth2client->getAuthenticatedRequest($method, $url, $token);
$response = $oauth2client->getResponse($request, $options);
```