Don't execute the transition if we're already in the final state

[jduff]

Problem

If you trigger a leave transition when you are already hidden the transition still fires modifying the classes, even though the element is already hidden.

Solution

Check if the element already has the hidden class and only trigger the transition if it isn't already in that state.

[mmccall10]

Sorry for the delay! Thank for this and #2. As you can see this does not have proper testing setup. I will take a look shortly.