fallbacks_test.rb never runs

[gtd]

The tests are there, but the condition to run them is never true. I added:

I18n::Backend::Simple.send(:include, I18n::Backend::Fallbacks)

And there are 4 failures. Obviously it's very hard to enable Fallbacks within a test suite since it's an included module. However these tests should be run, and I'm working on functionality that needs specific testing in fallbacks mode, so it'd be nice to work out some kind of automated testing here... maybe separate rake tasks?