Plug :assign and :unassign push options

app/services/merge_requests/push_options_handler_service.rb

@@ -139,6 +139,9 @@ module MergeRequests
       params[:add_labels] = params.delete(:label).keys if params.has_key?(:label)
       params[:remove_labels] = params.delete(:unlabel).keys if params.has_key?(:unlabel)
 
+      params[:add_assignee_ids] = params.delete(:assign).keys if params.has_key?(:assign)
+      params[:remove_assignee_ids] = params.delete(:unassign).keys if params.has_key?(:unassign)
+
       params
     end