Fix Gaufrette storage : metadata key with path + force overwrite

I've change the key argument in setMetadata to reflect your refactorization in order to get the same behaviour as #338 

And I add the third argument as true in filesystem->write() to tell to overwrite if a file with a same name is already present.
Don't know if you want this to be a configuration choice, it seems to me that it should always overwrite as a classic upload operation.