Close connections unless the header says otherwise

[cjcodes]

Close HTTP connections when done with a request.

This resolves an issue of running Twilio in AWS Lambda, where clients get a socket hang up.

For folks that don't want this additional header and would prefer to recycle connections, a custom implementation of RequestClient can be passed to Twilio via the httpClient option.

const t = new Twilio(sid, token, {
  httpClient: new MyOwnRestClientImpl(),
});