Update fixture class name deprecation warning to give more information about caller

I buried way too much time into a 4.1.0.beta1 upgrade trying to figure out why I was getting this deprecation warning:

> The ability to pass in strings as a class name will be removed in Rails 4.2, consider using the class itself instead.

After lots of debugging caller stacks, it turns out it comes from passing a class-name-as-string into `set_fixture_class` specifically (or `FixtureSet.new`, but I believe that's only used internally).

This just updates the copy of the deprecation warning to hopefully save some other poor souls some time.