def distinguished_folder_id!(dfid, change_key = nil, act_as = nil)
      attribs = {'Id' => dfid.to_s}
      attribs['ChangeKey'] = change_key if change_key
      @nbuild[NS_EWS_TYPES].DistinguishedFolderId(attribs) {
        if ! act_as.nil?
          mailbox!({:email_address => act_as})
        end
      }
    end
def distinguished_folder_id!(dfid, change_key = nil, act_as = nil)
      attribs = {'Id' => dfid.to_s}
      attribs['ChangeKey'] = change_key if change_key
      @nbuild[NS_EWS_TYPES].DistinguishedFolderId(attribs) {
        if ! act_as.nil?
          mailbox!({:email_address => act_as})
        end
      }
    end
Build the DistinguishedFolderId element
 @see http://msdn.microsoft.com/en-us/library/aa580808.aspx
 @todo add support for the Mailbox child object