Ensure composite wrapper type reflects reality

When a Supplier<Flux<Foo>> is composed with a Function<Foo,Bar>
the resulting handler (supplier) should have Flux as its output
wrapper still (the most general output wrapper type in the chain).

spring-cloud-function-samples/function-sample-pojo/src/test/java/com/example/SampleApplicationTests.java

@@ -51,6 +51,13 @@ public class SampleApplicationTests {
 				String.class)).isEqualTo("[{\"value\":\"FOO\"}]");
 	}
 
+	@Test
+	public void composite() {
+		assertThat(new TestRestTemplate()
+				.getForObject("http://localhost:" + port + "/words,uppercase", String.class))
+						.isEqualTo("[{\"value\":\"FOO\"},{\"value\":\"BAR\"}]");
+	}
+
 	@Test
 	public void single() {
 		assertThat(new TestRestTemplate().postForObject(