get host () {
    const proxy = this.app.proxy
    let host = proxy && this.get('X-Forwarded-Host')
    if (!host) {
      if (this.req.httpVersionMajor >= 2) host = this.get(':authority')
      if (!host) host = this.get('Host')
    }
    if (!host) return ''
    return splitCommaSeparatedValues(host, 1)[0]
  }
Parse the "Host" header field host
and support X-Forwarded-Host when a
proxy is enabled.

@return {String} hostname:port
@api public