Make AutocompleteModel compatible with Django 1.3.

Before QueryDict.getlist() only accepted one parameter. Compare
https://docs.djangoproject.com/en/1.3/ref/request-response/#django.http.QueryDict.getlist
with
https://docs.djangoproject.com/en/1.4/ref/request-response/#django.http.QueryDict.getlist

I have tested this in Django 1.3 and it's working, but it's not tested in Django 1.4 or 1.5.