iOS8.1.3下可能会出现userInfo中UIKeyboardFrame前后一样的情况

[yhb329]

在iPhone 5s iOS8.1.3系统版本下UIKeyboardWillChangeFrameNotification通知中有时候获取userInfo后UIKeyboardFrameBeginUserInfoKey&UIKeyboardFrameEndUserInfoKey，UIKeyboardCenterBeginUserInfoKey&UIKeyboardCenterEndUserInfoKey前后一样的情况，
会导致keyboardChangedWithTransition执行不到的问题。