def xml_http_request( url, props, headers = nil)
      # xhr doesn't seem to work because it's setting text/html before */xml in Accept header
      # also, it uses IndifferentAccess.merge which overrides any Accept header I try to send in!
      #xml_http_request '/omis/emails', props, 'Accept' => 'application/xml, text/xml'
      post url, props,
        (headers || {}).merge( 'X-Requested-With' => 'XMLHttpRequest',
        'Accept' => 'application/xml, text/xml')

      if( @http_basic_authentication_credentials_sent)
        if( @http_basic_authentication_credentials_are_correct)
          assert_response :success
        else
          assert_response 401
        end
        # TODO: validate return structure
      else
        assert_response 401 # HTTP basic authentication challenge
        assert_equal "Basic realm=\"user.joyomi.com\"", response.headers["WWW-Authenticate"]
        @http_basic_authentication_credentials_sent = true
        # repeat request with Authorization header
        xml_http_request( url, props, 'Authorization' => 'Basic ' + Base64.encode64(@username + ':' + @password)) #recurse
      end
    end