Issue 58 shouldbeoneof

Sorry about the line endings, I've got core.autocrlf set to true but my git-fu is too weak to figure out the problem.

I was just using enums as an example. This should be pretty generic. The error message is fairly understandable. I had to make a slight change to `DifferenceHighlighter` so it didn't get picked up and throw an `InvalidCastException` with the `actual` not being `IEnumerable`.