code: function(url, properties, headers) {
          headers = headers || {};
  
          headers['Content-Type'] = 'application/xml; charset=utf-8';
  
          var body =
              '<?xml version="1.0"?>\n' +
              '<d:propertyupdate ';
          var namespace;
          for (namespace in this.xmlNamespaces) {
              body += ' xmlns:' + this.xmlNamespaces[namespace] + '="' + namespace + '"';
          }
          body += '>\n' + this._renderPropSet(properties);
          body += '</d:propertyupdate>';
  
          return this.request('PROPPATCH', url, headers, body).then(
              function(result) {
                  return {
                      status: result.status,
                      body: result.body,
                      xhr: result.xhr
                  };
              }.bind(this)
          );
  
      }
docs: Generates a propPatch request.
  
  @param {string} url Url to do the proppatch request on
  @param {Object.<String,String>} properties List of properties to store.
  @param {Object} [headers] headers
  @return {Promise}