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/852375/1876333/d1ff5b6a-790f-11e3-9715-f682acfc9979.gif)

Notice the scrollbar keeps shrinking and you can never scroll to the bottom.

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.

    pod ‘TPKeyboardAvoiding’, :git => ‘https://github.com/jamztang/TPKeyboardAvoiding.git’, :branch => ‘bug/infinite-scrolling’