gcloud.storage.connection.Connection.api_request defaults path to None, but doesn't handle it

https://github.com/GoogleCloudPlatform/gcloud-python/blob/master/gcloud/storage/connection.py#L165

If the 'path' argument is not passed, the generated URI is nonsensical.  We should either default to a sensible path ('/'), or else test for None and do something reasonable.  As an alternative, we could just make 'path' a required argument (as it is for 'build_api_url').
