fix infiniteScrollingView still appear when UIScrollView content size is shorter than screen

[hotogwc]

when you have a blank tableView, if you trigger a pullToRefresh, the infiniteScrollingView will appear under pullToRefresh view. it should not be there in this circumstance.

[LawrenceHan]

Hi I merged #260 code to https://github.com/LawrenceHan/SVPullToRefresh Because it make more sense to me. It will solve your problem. Let me know if it's not working in your project.