[BUG] highlight when triggering pan gesture in default scale not working

[liuxuan30]

For commit 1d53151

It turned out that below cause the problem:

if !_dataNotSet && _dragEnabled &&
    (!self.hasNoDragOffset || !self.isFullyZoomedOut || self.isHighlightPerDragEnabled)
{
    _isDragging = true
    ...
}

Why || self.isHighlightPerDragEnabled here? It will make the whole if condition always true and lead _isDragging to true. then

else if (recognizer.state == UIGestureRecognizerState.Changed)
{
    if (_isDragging)
    {
        ...
    }

will always goes in isDragging, not triggering the highlight code path.

[liuxuan30]

Have a fix ready. Correct me if I misunderstood or missed anything