Indicate that GCS adapter can overwrite file

[daohoangson]

This PR implements the interface \League\Flysystem\Adapter\CanOverwriteFiles for GoogleCloudStorageAdapter to optimize away a Filesystem::has call. It looks like this in \League\Flysystem\Filesystem:

        if ( ! $this->getAdapter() instanceof CanOverwriteFiles && $this->has($path)) {
            return (bool) $this->getAdapter()->updateStream($path, $resource, $config);
        }

[daohoangson]

@cedricziel, can you take a look?

[cedricziel]

Thank you for your contribution, @daohoangson ! Sorry for being late :/

[daohoangson]

Thank you for merging! @cedricziel