set etag (val) {
    if (!/^(W\/)?"/.test(val)) val = `"${val}"`
    this.set('ETag', val)
  }
Set the ETag of a response.
This will normalize the quotes if necessary.

    this.response.etag = 'md5hashsum';
    this.response.etag = '"md5hashsum"';
    this.response.etag = 'W/"123456789"';

@param {String} val
@api public