Fixed the calculation of content Inset when using the method insertRowAtIndexPath: when keyboard is visible

There's a bug when inserting a row on a tableview when the keyboard is visible. The method TPKeyboardAvoiding_contentInsetForKeyboard miscalculate the bottom inset returning a wrong value. The wrong value is due the (CGRectGetMaxY(keyboardRect) - CGRectGetMaxY(self.bounds) returns a negative value. I don't know if this can return negative values but it fixes the bug in my case.