Alternative check on response type of client.userinfo_signed_response_alg

[adamchengtkc]

https://github.com/panva/oauth4webapi/blob/09bc1cd4434ed83888f30f154a2f15cea5f798df/src/index.ts#L3176C16-L3176C44

Suggest we use the same check as jwt to look for the expected return type (In this case application/json)

Reason:
Same OIDC provider would have the following in discovery when userinfo endpoint returns a JSON
"userinfo_signing_alg_values_supported":["none"]
In this case, the current setup will leads to throw OPE('JWT UserInfo Response expected', JWT_USERINFO_EXPECTED, response)

I can submit a PR to use conditional (getContentType(response) === 'application/json')

[panva]

I'm afraid I don't quite understand.