Fixes issue with Opera flickering on the collapse plugin when reset is called...

Fixes issue with Opera flickering on the collapse plugin when reset is called without specifying the size

https://github.com/twitter/bootstrap/issues/1208

js/bootstrap-collapse.js

@@ -71,7 +71,7 @@
 
       this.$element
         .removeClass('collapse')
-        [dimension](size || '')
+        [dimension](size || 'auto')
         [0].offsetWidth
 
       this.$element.addClass('collapse')