Fixed wrong calculation of bottomBoundary

If you try to log <code>isOverBottomBoundary</code> you can see that it is always false. It looks like bottomBoundary logic was wrong. 
The condition we actually need is <code>scrollView.height - scrollViewContent.height + offset > 0</code>. When this condition is true scroll view is over bottom boundary. I just translated this into variables and conditions in your code.

Also, I can't see reason why <code>_downThresholdY</code> is not 0 by default?