Fixed placeholder not updating after being set.
Fixed placeholder not updating after being set.

Fixes issue #68
alexeylang/GrowingTextView,Meihualu/GrowingTextView,zuojx1013/GrowingTextView,shenhzou654321/GrowingTextView,mabuward90/GrowingTextView,HansPinckaers/GrowingTextView,happy-tanuki/GrowingTextView,Kingside/GrowingTextView,ywzw2013/GrowingTextView