Fix a bug when scrollview has content inset top 0 

Using the refresh control with scrollview with content inset top 0 (can cause by ```self.edgesForExtendedLayout = UIRectEdgeNone;```) will cause original content inset to be set as the content inset after the refresh control has started animating -> it will not go back to content inset 0. You can test it easily by comment out the line to set ```self.edgesForExtendedLayout ``` in the sample project. To fix it, only set self.originalTopContentInset when state is still Idle (no pull refresh has occurred)