Restore visual diff to original behavior

This restores the following corner case:

When there is a difference, but it's only about
the case of the characters, don't want to show
the visual diff by default, but we offer the
feature to the user.

I also updated the tests the reflect the specs.
I don't see the need for the `else` here, you're checking if something is defined, then if it's not you set it to `undefined`?