handle korean keyboard properly

[0mok]

if we use korean keyboard, UITextView occasionally fires 2 textViewDidChange events simultaneously. for example, there is "버그버그ㅂ". we presse the "ㅓ" key. then text is changed to "버그버그버" but UITextView fires 2 textViewDidChange events. The first event is "버그버그ㅂ" to "버그버그". The second event is "버그버그" to "버그버그버". so we should use the last event of these events.

current version capture https://www.dropbox.com/s/j7jswke288ye6o5/GrowingTextView-Korean-Bug.mov

fixed version capture https://www.dropbox.com/s/2ehazrxkgbotq1h/GrowingTextView-Korean-Bug-Fixed.mov

[eincs]

+1 to this PR