Patch for review: improved reporting of failed expression in assert_difference

Hey guys.

First patch. Hopefully I'm going about this the right way :slight_smile:

Anyway, smallish patch to active_support/testing/core_ext/test/unit/
assertions.rb to provide more explicit messages on assertion failures.
Outputs the expression that failed as well as the expected and actual
value.

http://rails.lighthouseapp.com/projects/8994/tickets/1161-improved-reporting-of-failed-expression-in-assert_difference

cheers
madlep