Add support for moving stories between projects

lib/tracker_api/resources/story.rb

@@ -54,6 +54,7 @@ class UpdateRepresenter < Representable::Decorator
         property :deadline
         property :requested_by_id
         property :owner_ids, if: ->(_) { !owner_ids.blank? }
+        property :project_id
 
         # Use render_empty: false to address: https://github.com/dashofcode/tracker_api/issues/110
         # - The default value of the labels attribute in Resources::Story is an empty array.
@@ -165,6 +166,17 @@ def transitions(params = {})
         end
       end
 
+      # Returns the story's original ("undirtied") project_id
+      #
+      # @return Integer
+      def project_id
+        if dirty_attributes.key?(:project_id)
+          original_attributes[:project_id]
+        else
+          @project_id
+        end
+      end
+
       # @param [Hash] params attributes to create the task with
       # @return [Task] newly created Task
       def create_task(params)