fix bug:scroll to top trigger to scrolltobottom
fix bug:scroll to top trigger to scrolltobottom

lzwjava/leanchat-ios,leancloud/leanchat-ios,dujiepeng/leanchat-ios,lzwjava/leanchat-ios,lzwjava/leanchat-ios,tang3w/leanchat-ios,tang3w/leanchat-ios,dujiepeng/leanchat-ios,leancloud/leanchat-ios,wzf/leanchat-ios,leancloud/leanchat-ios,wzf/leanchat-ios