Fix for drop-auto-width collision detection and css border top

- collision detection was not moving the drop left based on the
difference between viewport and relative location of box (dropleft +
dropwidth)
- .select2-drop-active was overriding the .select2-drop-auto-width
making the shifted dropbox look a bit wierd
Merge pull request #1891 from jfisheratwork/master

Fix for collision detection regarding enoughRoomOnRight and css border top for drop-auto-width