fix api endpoints for storage upload/download

The problem is that I added `upload` to `build_api_url` in `Storage.Connection`; at this point:

* if I don't add it to the signature of this mock, the test fails when `build_api_url` is called for uploads passing in the parameter
* if I add it to the signature, but I don't use it somehow, I get the `unused argument` error from pylint (which could be solved by adding `# pylint:disable=unused-argument`)