Fix Apple key retrieval when using Guzzle logging (#39)

When using Guzzle logging to log a message body the stream is not reset to
the start, causing future calls to getContents() to be empty. It seems
recommended way of getting the body as a string is to use `__toString()`
(guzzle/guzzle#1842)

src/Provider/Apple.php

@@ -89,7 +89,7 @@ private function getAppleKeys()
  $response = $this->httpClient->request('GET', 'https://appleid.apple.com/auth/keys');
 
  if ($response && $response->getStatusCode() === 200) {
- return JWK::parseKeySet(json_decode($response->getBody()->getContents(), true));
+ return JWK::parseKeySet(json_decode($response->getBody()->__toString(), true));
  }
 
  return [];