Fix bug where cached unit test responses aren't serialised (geocoder-php#530)

Author: vicchi

tests/Geocoder/Tests/CachedResponseClient.php

@@ -44,7 +44,7 @@ public function sendRequest(RequestInterface $request)
  $response = $this->delegate->sendRequest($request);
 
  if ($this->useCache) {
- file_put_contents($file, $response->getBody()->getContents());
+ file_put_contents($file, serialize($response->getBody()->getContents()));
  }
 
  return $response;