The new API fields support wasn't really testing what we wanted to test

as far as string types go. It was testing specifically Unicode strings

vs. byte strings, which wasn't going to work on Python 3. It should have

been testing Unicode strings vs. native strings (which are the same

thing on Python 3, but not on 2.7).
This change fixes up the tests and also fixes up string normalization to

ensure that any byte strings are properly decoded to Unicode strings.
It also fixes some ImportError-related checks in the tests to properly

match error messages across Python 2.x and 3.x.

Relevant unit tests pass on Python 2.7, 3.4, 3.5, and 3.6.