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 think it would be nice to pull this code out into a method that returns the `hasDiff, showDiff` properties.

    {@showDiff, @hasDiff} = this.diffFromTarget(@annotation.target)