code: public MuServerBuilder addHandler(MuHandler handler) {
          if (handler != null) {
              handler = getContextualHandlerForResourceHandler(handler);
              handlers.add(handler);
          }
          return this;
      }
docs: <p>Adds a request handler.</p>
  <p>Note that handlers are executed in the order added to the builder, but all async
  handlers are executed before synchronous handlers.</p>
  @see #addHandler(Method, String, RouteHandler)
  @param handler The handler to add. If null, then no handler is added.
  @return The current Mu-Server Handler.