Default to auto width if container has 0 width (hidden)

select2.js

@@ -1033,7 +1033,7 @@
                         return matches[1];
                 }
             }
-            return this.opts.element.width() + 'px';
+            return (this.opts.element.width() === 0 ? 'auto' : this.opts.element.width() + 'px');
         }
     });