Should response/request.header[T] be ModeledCustomHeader aware?

This requires a bunch of hacks though, as the impl that makes it work in akka-http is the `HeaderMagnet` infra which we do not have immediately available in akka-http-core, and I'm not sure if we want to have it there.

https://github.com/akka/akka-http/blob/master/akka-http/src/main/scala/akka/http/javadsl/server/directives/HeaderDirectives.scala#L84

We can make it work using some smarts however making it possible for Java should likely use the partial function style as we suggest anyway for the routing DSL for it.

Start with akka-http/src/main/scala/akka/http/javadsl/server/directives/HeaderDirectives.scala and the referenced HeaderMagnet implementation in akka-http. Determine whether response/request.header[T] should support ModeledCustomHeader, and whether the Java API should follow the partial-function style suggested for the routing DSL. Done means a decided, documented approach for akka-http-core and Java usage.

Written by the indexing model from the issue text.