lib/ews/soap/builders/ews_builder.rb
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
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
Build the FolderIds element
 @see http://msdn.microsoft.com/en-us/library/aa580509.aspx