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.
The problem with doing that is that the calculation actually also sets some flags inside the targets, so naming the method like you suggested would be misleading. (It would indicate that this is a clear calculation, without side effects on the input data, but that's not the case.)