Update project milestone resource

Update project milestone resource with new attributes and the
ability to fabricate a milestone with or without start and due
date.

qa/qa/ee/resource/project_milestone.rb

@@ -4,7 +4,10 @@ module QA
   module EE
     module Resource
       class ProjectMilestone < QA::Resource::Base
+        attr_writer :start_date, :due_date
+
         attribute :id
+        attribute :iid
         attribute :title
 
         attribute :project do
@@ -30,7 +33,10 @@ module QA
         def api_post_body
           {
             title: title
-          }
+          }.tap do |hash|
+            hash[:start_date] = @start_date if @start_date
+            hash[:due_date] = @due_date if @due_date
+          end
         end
       end
     end