userpass auth method needs tests

[briantist]

SUMMARY

Related: #66

It seems like the userpass auth method may never have been working, at least not with the version that's in the newer auth class (I'm unsure on the deprecated method in hvac).

And there are no tests for userpass so we don't have any way to catch regressions or problems.

The userpass method is easily testable since it has no external dependencies. Putting this issue here to track the need.

This will probably get done as part of the refactor project, when the auth method moves centrally.

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