Fixes two broken tests, issue #142, and enables tests now they're fixed

[lisad]

The tests are pretty descriptive:

• if we're fixing column values (by applying consistent capitalization, stripping spaces, etc) that should happen before we see if the value is in an enumerated list of possible valeus
• The enumerated list of possible values should be a list! otherwise we can wrap it in one and assume the programmer meant for it to be a single value.

Fixes #142

[lisad]

@YuliaS , I incorporated all of your tests except for the ones that try to apply a "fix_value_fn" that only works before the Column casts to a type.

To address those, I instead added some documentation that explains that fix_value_fn applies after casting to the declared type, so 'upper' and 'strip' won't work on an IntColumn or DateColumn, but contrariwise, 'abs' will work on an IntColumn because it's already cast to an int.