toHTTP() {
    return toTechFormat(this.toUTC(), "EEE, dd LLL yyyy HH:mm:ss 'GMT'");
  }
Returns a string representation of this DateTime appropriate for use in HTTP headers. The output is always expressed in GMT.
Specifically, the string conforms to RFC 1123.
@see https://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3.1
@example DateTime.utc(2014, 7, 13).toHTTP() //=> 'Sun, 13 Jul 2014 00:00:00 GMT'
@example DateTime.utc(2014, 7, 13, 19).toHTTP() //=> 'Sun, 13 Jul 2014 19:00:00 GMT'
@return {string}