Allow http/2 requests to fallback to host header if authority pseudoheader is not provided

[j-white]

Describe the feature

Some HTTP/2 clients omit the :authority header in favor of the Host header.

According to the HTTP/2 specification, the :authority SHOULD be included, but is not necessarily required.

Use cases

One notable client with this behaviour is the AWS ALB. Even if it is an HTTP/2 request, the host header is given and the :authority header is not. See references for details.

References

• Allow http/2 requests to fallback to host header if authority pseudoheader is not provided quarkusio/quarkus#37531
• Hostname is not processed correctly when accepting HTTP/2 requests in KC_PROXY=edge mode keycloak/keycloak#20959
• Server should look for "host" header field if ":authority" pseudo-header field is not present grpc/grpc#1022