fix: allow specifying the user-agent header for outgoing requests

[pvieira-diligent]

Specifying the user-agent header for outgoing requests was introduced in 95f24ef.

This PR is fixing an issue with that commit: the user-agent header value set in the httpOptions config is not really being used, and the http agent defaults to not sending the user-agent HTTP header.

  const helperOptions = pickBy({ signal, agent, dnsLookup }, Boolean);
  // ...
  options.headers['user-agent'] = helperOptions['user-agent']; // at this point, helperOptions['user-agent'] is always undefined

In practice, due to the missing user-agent, WAFs could block requests to the jwks_uri endpoint during a client authentication.

[panva]

Clearly i should've added a test then, apologies. Can you add a test now?

[pvieira-diligent]

Clearly i should've added a test then, apologies. Can you add a test now?

I've added a couple of tests already.

Are you expecting different ones? I'm happy to add more if I'm missing any scenarios.

[panva]

Thank you @pvieira-diligent