Always unpin if scroll exceeds offset

Is there a reason the offset behavior of shouldUnpin doesn't match shouldPin?

This change forces the header to unpin if you scroll down past the offset, which can be useful in some cases. For instance, you can set a very large tolerance, so the header only changes at the offset boundary. We're using this to create a stacked set of headers similar to Google+, where the top header collapses and the subheader becomes pinned as you scroll.