BugFix: Prefer ajax over sAjaxSource

According to a comment in https://github.com/DataTables/DataTablesSrc/blob/master/js/api/api.ajax.js#L121, `ajax` has priority over `sAjaxSource`.

This is not true in https://github.com/DataTables/DataTablesSrc/blob/master/js/core/core.ajax.js#L103 if `fnServerData` is set, where only `sAjaxSource` is used.

I ran into this bug in combination with https://github.com/l-lin/angular-datatables, where even after changing the `sAjaxSource` property the initial url is used in all subsequent calls to `fnServerData`.