use HeaderCollection::toArray to fix lowercased header names

When creating a Response in Goutte client from a GuzzleResponse, headers names are lowercased.
(location, content-type instead of Location, Content-type).

(see https://github.com/guzzle/guzzle/blob/master/src/Guzzle/Http/Message/Header/HeaderCollection.php#L44)

Rather than use `getAll` method of `HeaderCollection`, we better use [`toArray`](https://github.com/guzzle/guzzle/blob/master/src/Guzzle/Http/Message/Header/HeaderCollection.php#L99).