add validations for boolean attributes

spec/models/milestone_spec.rb

@@ -9,6 +9,7 @@ describe Milestone do
   describe "Validation" do
     it { should validate_presence_of(:title) }
     it { should validate_presence_of(:project_id) }
+    it { should ensure_inclusion_of(:closed).in_array([true, false]) }
   end
 
   let(:milestone) { Factory :milestone }