Removing class which is currently animating doesn't work.

Angular Version: since 1.4.5 till 1.5.0-build.4389
Browser: Any
Reproducable: almost always (sometimes doesn't reproduce under debugging)
Operating system: Any

Detail
For the example of bug see plunker
By normal it should not have class pending at the end.

What happens:

1. Adding class pending
2. NgModel adds class ng-valid-required (it's joining animation and applies preparation class)
3. Timeout is fullfilled
4. Trying to remove class.
   Inside ngAnimations it checks that element has animations, it cannot be cancelled and skipped, so it's try to join animation. Because element is in running state it tries to normalize options. Function normalize options cannot find class pending and remove it from options. Remove animation exits as invalid.
5. Class pending added to dom node