Use the same performance special case for 'country' as well as 'type'

Presumably the same fix for odd PostGIS query planning needs to be done
for the new 'country' filter as for 'type', since these GET query
parameters modify the shape of the query in the same way.

mapit/views/areas.py

@@ -364,8 +364,9 @@ def areas_by_point(request, srid, x, y, bb=False, format='json'):
 
     args = query_args(request, format)
     type = request.GET.get('type', '')
+    country = request.GET.get('country', '')
 
-    if type and method == 'polygon':
+    if (type or country) and method == 'polygon':
         args = dict(("area__%s" % k, v) for k, v in args.items())
         # So this is odd. It doesn't matter if you specify types, PostGIS will
         # do the contains test on all the geometries matching the bounding-box