Fix `dropdownAutoWidth` so it actually works

Previously the `dropdownAutoWidth` option didn't work because the
dropdown was being absolutely positioned, so setting the width to
`auto` didn't actually have an effect. This cannot be solved by
just setting the width on the container, because that is also
absolutely positioned.

This closes https://github.com/select2/select2/issues/3101

src/js/select2/dropdown/attachBody.js

@@ -204,6 +204,7 @@ define([
 
     if (this.options.get('dropdownAutoWidth')) {
       css.minWidth = css.width;
+      css.position = 'relative';
       css.width = 'auto';
     }