Refactor project board list resources

... to remove duplications.

qa/qa/ee/resource/board/board_list/project/assignee_board_list.rb

@@ -10,10 +10,9 @@ module QA
               attribute :assignee
 
               def api_post_body
-                {
-                  board_id: board.id,
+                super.merge({
                   assignee_id: assignee.id
-                }
+                })
               end
             end
           end

qa/qa/ee/resource/board/board_list/project/label_board_list.rb

@@ -15,10 +15,9 @@ module QA
               end
 
               def api_post_body
-                {
-                  board_id: board.id,
+                super.merge({
                   label_id: label.id
-                }
+                })
               end
             end
           end

qa/qa/ee/resource/board/board_list/project/milestone_board_list.rb

@@ -15,10 +15,9 @@ module QA
               end
 
               def api_post_body
-                {
-                  board_id: board.id,
+                super.merge({
                   milestone_id: project_milestone.id
-                }
+                })
               end
             end
           end