text: function(eventCollection) {
    var onHeadersReceivedEvent = eventCollection.firstEventOfType('onHeadersReceived');
    var { responseHeaders } = onHeadersReceivedEvent;
    var encoding = responseHeaders['content-encoding'];
    return !encoding || encoding !== 'gzip' ? this.createResult(5, 'HTTP', `Content-Encoding: ${encoding}`, 'warning') : null;
  }
  