Motivation:
We store HTTP/2 pseudo headers to the map in `HttpHeaders` with other headers.
Then, the entries of the map are copied to Spring `HttpHeaders` in Spring integration.
However, it can be a problem when the copied Spring `HttpHeaders` is converted to Netty Headers later:
https://github.com/spring-cloud/spring-cloud-gateway/blob/v3.1.3/spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/filter/NettyRoutingFilter.java#L125-L128
The validation is failed when the pseudo headers are set.

Modifications:
- Remove HTTP/2 pseudo headers when creating Spring `HttpHeaders` from Armeria `HttpHeaders`.
  - Convert `:authority` header to `HOST` header.

Result:
- You no longer see `IllegalArgumentException` indicating prohibited character of a header name
  in an environment that Spring integration module and Netty client are used togeter.(e.g. Spring Cloud Gateway)