external_auth: handle request.META values as str, not unicode

@nedbat @marcore

This is the more systematic shib changes we talked about on https://github.com/edx/edx-platform/pull/5034
Basically, it fixes the test cases for `request.META` to generate `str`s with unicode points instead of `unicode`.  And it converts values read from `request.META` to `unicode` as soon as possible.

@marcore do you want to see if these changes also solve your issues?