Added some implementation for overflow of month and year text fields.

[DannyVancura]

@JamesJSY

• Instead of having autocompletedNewText.characters.count > expectedDigits inside an if-else block, it is additional to the following if-block: If there is an overflow of characters, the DetailInputTextField's delegate will be notified about this first and then the validation takes place.
• Instead of setting nextTextField's text directly, rely on the existing logic in textField(_, shouldChangeCharactersInRange:_, replacementString:_) to change the text.

[JamesJSY]

Made protocols and overflow checking self-explanatory