fix demo - scrollViewDidScroll will get called when view is not visible
fix demo - scrollViewDidScroll will get called when view is not visible

GitHubChenFeng/LTNavigationBar,200895045/LTNavigationBar,KBvsMJ/LTNavigationBar,WMSmile/LTNavigationBar,agan112/LTNavigationBar,OnTheWay1988/LTNavigationBar,ondev/LTNavigationBar,zdw19840929/LTNavigationBar,iostalks/LTNavigationBar,zilaiyedaren/LTNavigationBar,ShengQiangLiu/LTNavigationBar,wonderffee/LTNavigationBar,msdgwzhy6/LTNavigationBar,joblh/LTNavigationBar,ltebean/LTNavigationBar,docee/LTNavigationBar,TiagoVeloso/LTNavigationBar,UncleJoke/LTNavigationBar