get (field) {
    const req = this.req
    switch (field = field.toLowerCase()) {
      case 'referer':
      case 'referrer':
        return req.headers.referrer || req.headers.referer || ''
      default:
        return req.headers[field] || ''
    }
  }
Return request header.

The `Referrer` header field is special-cased,
both `Referrer` and `Referer` are interchangeable.

Examples:

    this.get('Content-Type');
    // => "text/plain"

    this.get('content-type');
    // => "text/plain"

    this.get('Something');
    // => ''

@param {String} field
@return {String}
@api public