Fix the reset to CGAffineTransformIdentity not being animated

When reseting the text field to its original position with `CGAffineTransformIdentity`, the change is not animated, which makes the text field "jump" at the end of the animation.

Before fix: https://dl.dropboxusercontent.com/u/18386288/UIView%2BShake_before.mov
After fix: https://dl.dropboxusercontent.com/u/18386288/UIView%2BShake_after.mov

(sorry, .mov videos, could not find a satisfying gif converter...)