fix to allow distinct data-ajax-url attributes on individual select2 widgets

Because a shared options object is passed around when creating each
instance of a select2 widget, the query function that was built would
use the last ajax url that was set on the options object for all select2
widgets. If the ajax url is set via the data-ajax-url then each select item should be allowed to have a distinct url for fetching options. Currently the last url encountered is used by all calls.