WIP: Fix problem with UUIDs being case-insensitive

[booch]

This is a fix for issue #962.

[mcmire]

Thanks for this PR. A couple of things:

• You might want to update column_for so it makes use of column_for_attribute instead of columns_hash, that way you can use it instead of calling column_for_attribute manually.
• Would be great to have a test for this! We already have a test for an integer column here: https://github.com/thoughtbot/shoulda-matchers/blob/master/spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb#L362. You might be able to base your test off of that.

[booch]

Thanks for the feedback, @mcmire. I've got a simple acceptance test in my actual Rails project, but I've had a hard time figuring out how to write the test with the testing framework in place. If I continue to struggle with that, I'll probably try to coordinate some time to pair up with someone to get it knocked out.