Break down new topic counts by category if a digest contains many.

spec/mailers/user_notifications_spec.rb

@@ -52,6 +52,7 @@ describe UserNotifications do
     context "with new topics" do
       before do
         Topic.expects(:for_digest).returns([Fabricate(:topic, user: Fabricate(:coding_horror))])
+        Topic.expects(:new_since_last_seen).returns(Topic.none)
       end
 
       its(:to) { should == [user.email] }