Fix #354: support slash in key names

Quote 'key.name' properly when used in URL path / query string.

Fixes #354.
Break `test_upload_from_file_w_slash_in_name` into more tests or use helper methods to deal with locals and lines issues.

As for instance attributes, see my [previous comment][1].

[1]: https://github.com/GoogleCloudPlatform/gcloud-python/pull/358#commitcomment-8503977