fix(collapse): fix occasional flickering

- Fixes occasional flickering that can happen

Closes #3804
Fixes #3801

Author: mseancole

src/collapse/collapse.js

@@ -17,6 +17,10 @@ angular.module('ui.bootstrap.collapse', [])
         }
 
         function collapse() {
+          if(! element.hasClass('collapse') && ! element.hasClass('in')) {
+            return collapseDone();
+          }
+
           element
             // IMPORTANT: The height must be set before adding "collapsing" class.
             // Otherwise, the browser attempts to animate from height 0 (in