'Assigned to' drop down list is now sorted by user's lastname.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@939 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/user.rb

@@ -163,7 +163,7 @@ class User < ActiveRecord::Base
   end
 
   def <=>(user)
-    lastname == user.lastname ? firstname <=> user.firstname : lastname <=> user.lastname
+    user.nil? ? -1 : (lastname == user.lastname ? firstname <=> user.firstname : lastname <=> user.lastname)
   end
   
   def to_s