@Override
    public void putAll(Map<? extends String, ? extends List<String>> m) {
        h.putAll(m);
    }
A multi-map for Request and Response header fields.

@see Request
@see Response
@author Einar M R Rosenvinge
@author Simon Thoresen Hult
application/src/main/java/com/yahoo/application/container/handler/Headers.java