Prevent header transform start when user drags content down

Before this change **prevScrollTop** could be set as negative value (user drags content down on bouncing view), and  **transformHeader** was executed to earlier and this was causing that unwanted gap was created between header and content.

I think that video explains it (iOS Simulator Screecast):

before change: https://www.youtube.com/watch?v=oonaB_0hEH4
after change: https://www.youtube.com/watch?v=srR2RjqhDGY