Django can end up giving us a buffer (Python 2.x) or memoryview

(Python 3.x) value when instantiating a field from a row in the

database. This wasn't supported in Base64Field, and led to breakages

in one part of the administration UI.
This extends Base64Field to work correctly with a six.memoryview

(which maps to the correct type on both versions of Python). Unit tests

ensure that all the type-specific behavior will work correctly when

provided with these values.

Unit tests pass on all supported versions of Python.
Verified that I was able to load the objects in the administration UI

that were triggering this bug.