display milestones that are not marked with a due date as active

app/models/milestone.rb

@@ -6,7 +6,7 @@ class Milestone < ActiveRecord::Base
   validates_presence_of :title
 
   def self.active
-    where("due_date > ? ", Date.today)
+    where("due_date > ? OR due_date IS NULL", Date.today)
   end
 
   def percent_complete