HttpMessageComposer#decompose should automatically try to transform message content to String. Otherwise you get status HttpServletResponse.SC_BAD_GATEWAY.

User has to add custom message composer now, e.g.

public class CustomHttpMessageComposer extends HttpMessageComposer {

	@Override

	public HttpBindingData decompose(Exchange exchange, HttpBindingData target) throws Exception {

		exchange.getMessage().setContent(

				exchange.getMessage().getContent(String.class)

		);

		return super.decompose(exchange, target);

	}

}

I would check if current object is instance of expected output and if so I would transform the content to String.