Improve querystring filtering.

This changed excludes more querystring parameters and relies on Django's
knowledge of what keys should be excluded. It basically does the same
what the ChangeList class would do [1].

[1] https://github.com/django/django/blob/master/django/contrib/admin/views/main.py#L91-L102

adminsortable/admin.py

@@ -6,6 +6,7 @@ from django.conf import settings
 from django.conf.urls import url
 from django.contrib.admin import ModelAdmin, TabularInline, StackedInline
 from django.contrib.admin.options import InlineModelAdmin
+from django.contrib.admin.views.main import IGNORED_PARAMS
 from django.contrib.contenttypes.admin import (GenericStackedInline,
                                                GenericTabularInline)
 from django.contrib.contenttypes.models import ContentType
@@ -38,8 +39,8 @@ class SortableAdminBase(object):
         filters = {}
 
         for k, v in request.GET.items():
-            if k not in ['_to_field', '_popup',]:
-                filters.update({ k: v })
+            if k not in IGNORED_PARAMS:
+                filters[k] = v
 
         return filters