Keyboard avoiding not working properly on Collection Views

There's an infinite scrolling bug when using TPKeyboardAvoiding with UICollectionView.

![keyboard-avoid](https://f.cloud.github.com/assets/852375/1876169/700192dc-790c-11e3-9e47-3a9e72a5ec72.gif)

It's caused by repetitive calling `setContentSize:` while scrolling, which doesn't happen in UITableView or UIScrollView.

The fix is to prevent setting the same value when the contentSize is already properly configured.