Convert Consumer<Foo> to Function<Flux<Foo>,Mono<Void>>

This results in a better experience for users because the consumer
that they write is only applied to a Flux that is subscribed to
by the framework once. It gives better control over the flow of
foos, e.g. if some component wants to subscribe on a thread.

spring-cloud-function-samples/function-sample-compiler/src/test/java/com/example/SampleCompiledConsumerTests.java

@@ -44,8 +44,7 @@ public class SampleCompiledConsumerTests {
 	@Test
 	public void print() {
 		assertThat(new TestRestTemplate().postForObject(
-				"http://localhost:" + port + "/test", "it works", String.class))
-						.isEqualTo("it works");
+				"http://localhost:" + port + "/test", "it works", String.class)).isNull();
 		assertEquals("it works", Reference.instance);
 	}