`@ResponseContentType` annotation limit

At the moment `@ResponseContentType` support specify content type by [`Http.MediaType`](https://github.com/osglworks/java-http/blob/osgl-http-1.13.3/src/main/java/org/osgl/http/H.java#L1783) enum only, if app developer needs to specify a Content-Type on the response that is not defined in the `MediaType` enum, they can't do anything even to workaround it.