Save mail auth info as plain strings, not unicode strings.

The HMAC library shipped with newer Python 2.6 releases doesn't work with
Unicode strings. The library really is intended to work with bytes, and
Python 3 apparently enforces this more heavily. We were getting the mail
username and password as a Unicode string from the form and then saving it
to settings as a Unicode string, which eventually made it to the HMAC library
and failed.

We now deserialize this as a plain string instead of keeping the default
behavior of deserializing as a Unicode string. This fixes the incompatibility
with the HMAC library.

Added a unit test to check for this behavior.

The unit test failed the HMAC test (and the type(...) == str check, but I temporarily removed that for verification) before the fix to these settings. After the fix, the unit test passed.