miniswift: refactor and test

Please note: This change is a derivative of https://github.com/zerovm/zpm/pull/65.

This change adds some minor tweaks to the innner workings `miniswift.SwiftClient` class, as well as some tests for the `auth` method. With these changes, test coverage of `miniswift` jumps from 22% to 69% (overall coverage goes from 34% to about 45%).

I plan to follow this patch up with more tweaks and tests to improve that number even further. My goal is to have >80% coverage, overall.