fixed error where mobile menu didn't expand

The first time expanding the mobile menu in bootstrap, submenus would not make the menu taller - this resulted in long menus losing entries.
This issue could be resolved by closing and opening the menu.
I found that changing the dimension on the initial open function to 'auto' fixed this issue, and didn't seem to have negative effects.
Tested in Chrome, Firefox, and Safari under OS X 10.8.2; tested in IE7, IE8, and IE9 under Windows 7.