Navbar collapsed dropup caret should point down

Created by: Hopper262

When in collapsed mode, a navbar's dropdowns and dropups behave identically (opening down). However, the triangle indicator still reflects the non-collapsed behavior, which for dropups, points away from the content.

Here's a test case on alpha 6, based on one of the documentation examples: https://jsfiddle.net/2htxfvz1/