adds missing priority_id to planning_element/show in API v2

app/views/api/v2/planning_elements/_planning_element.api.rabl

@@ -31,7 +31,7 @@
 # which drastically affects the performance of inline vs. partials: as the planning-elements are
 # highly performance-critical, we need to live with this duplication until these issues are solved.
 object @planning_element
-attributes :id, :subject, :description, :project_id, :parent_id, :status_id, :type_id
+attributes :id, :subject, :description, :project_id, :parent_id, :status_id, :type_id, :priority_id
 
 node :start_date, :if => lambda{|pe| pe.start_date.present?} { |pe| pe.start_date.to_formatted_s(:db) }
 node :due_date, :if => lambda{|pe| pe.due_date.present?} {|pe| pe.due_date.to_formatted_s(:db) }