admin: use BitHandler instead of bitor operator

Fixes disqus#61, disqus#64, and disqus#89 in BitFieldListFilter,
with a more efficient implementation.

bitfield/admin.py

@@ -6,8 +6,7 @@
 from django.contrib.admin import FieldListFilter
 from django.contrib.admin.options import IncorrectLookupParameters
 
-from bitfield import Bit
-from bitfield.compat import bitor
+from bitfield import Bit, BitHandler
 
 
 class BitFieldListFilter(FieldListFilter):
@@ -24,9 +23,14 @@ def __init__(self, field, request, params, model, model_admin, field_path):
             field, request, params, model, model_admin, field_path)
 
     def queryset(self, request, queryset):
-        filter = dict((p, bitor(F(p), v)) for p, v in six.iteritems(self.used_parameters))
+        filter_kwargs = dict(
+            (p, BitHandler(v, ()))
+            for p, v in six.iteritems(self.used_parameters)
+        )
+        if not filter_kwargs:
+            return queryset
         try:
-            return queryset.filter(**filter)
+            return queryset.filter(**filter_kwargs)
         except ValidationError as e:
             raise IncorrectLookupParameters(e)