code: def folder_ids!(fids, act_as=nil)
        ns = @nbuild.parent.name.match(/subscription/i) ? NS_EWS_TYPES : NS_EWS_MESSAGES
        @nbuild[ns].FolderIds {
          fids.each do |fid|
            fid[:act_as] = act_as if act_as != nil
            dispatch_folder_id!(fid)
          end
        }
      end
docs: Build the FolderIds element
   @see http://msdn.microsoft.com/en-us/library/aa580509.aspx