OIDC JWK endpoint expects application/jwk-set+json but only gets accept application/json (406 Not Acceptable)

[frederik]

Describe the bug

The jwk endpoint to retrieve the JWK sets seems to usually accept application/json as an accept header.
However, some OIDC provider implementations explicitly expect Accept: 'application/jwk-set+json' (which is valid for JWKS according to the spec). As a result they send back a 406 Not Acceptable.

Other libraries seem to have added this header, see Tomcat OIDC auth and others.

I do not have access to a public endpoint with this behavior, but would be able to verify once the header is added.

Expected behaviour

Setting the headers to Accept: 'application/json, application/jwk-set+json'
results in a successful request.

The line responsible is the GET request in function getKeyStore (lib/helpers/issuer.js). The following headers result in a successful request

headers: {
    Accept: 'application/json, application/jwk-set+json',
}

I'd be happy to submit a pull request, if this is accepted as a bug.

Environment:

• openid-client version: 5.1.1
• node version: 16.13.1

Additional context
Add any other context about the problem here.

• the bug is happening on latest openid-client too.
• i have searched the issues tracker on github for similar issues and couldn't find anything related.

Thanks for this great library and the work you put into it 😃 !

[panva]

Feel free to open a PR, I'll have a look at it!