get protocol () {
    if (this.socket.encrypted) return 'https'
    if (!this.app.proxy) return 'http'
    const proto = this.get('X-Forwarded-Proto')
    return proto ? splitCommaSeparatedValues(proto, 1)[0] : 'http'
  }
Return the protocol string "http" or "https"
when requested with TLS. When the proxy setting
is enabled the "X-Forwarded-Proto" header
field will be trusted. If you're running behind
a reverse proxy that supplies https for you this
may be enabled.

@return {String}
@api public