INT-3477: Add Reactor Promise<?> for Gateway

JIRA: https://jira.spring.io/browse/INT-3477
Adding a `flush()` in the `GatewayProxyFactoryBean` makes the flow run like with a `Future<?>` but it's not clear from the javadoc that it's the right thing to do.

		if (Promise.class.isAssignableFrom(returnType)) {
			return Promises.<Object>task(Functions.supplier(new AsyncInvocationTask(invocation)))
					.env(this.reactorEnvironment)
					.get()
					.flush();
		}

	/**
	 * Flush any cached or unprocessed values through this {@literal Stream}.
	 *
	 * @return {@literal this}
	 */

cc/ @jbrisbin @smaldini