Fixed cache key

The request's body is not correctly serialized since it's a stream, so the key can be the same for requests having just a different body.
Type casting to string resolves the problem.

Author: rpg600

src/GuzzleHttp/Cache/DoctrineAdapter.php

@@ -46,7 +46,7 @@ private function getKey(RequestInterface $request)
             'method'  => $request->getMethod(),
             'uri'     => $request->getUrl(),
             'headers' => $request->getHeaders(),
-            'body'    => $request->getBody(),
+            'body'    => (string) $request->getBody(),
         ]));
     }
 }