Add support for Message handling in web functions

If the output is a Message we grab the headers from the first one
and send those as response headers.

A function can add headers to messages. The HTTP response will
contain only headers that are in x-* _or_ were added to the message
by user (i.e. they weren't in the request).
We certainly do not want to pass through all HTTP headers (request
headers can conflict with or invalidate response headers).

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/web/RestApplicationTests.java

@@ -45,6 +45,8 @@ import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.util.StringUtils;
 
@@ -256,6 +258,26 @@ public class RestApplicationTests {
 		assertThat(result.getBody()).isEqualTo("[\"(FOO)\",\"(BAR)\"]");
 	}
 
+	@Test
+	public void messages() throws Exception {
+		ResponseEntity<String> result = rest.exchange(RequestEntity
+				.post(new URI("/messages")).contentType(MediaType.APPLICATION_JSON)
+				.header("x-foo", "bar").body("[\"foo\",\"bar\"]"), String.class);
+		assertThat(result.getBody()).isEqualTo("[\"(FOO)\",\"(BAR)\"]");
+		assertThat(result.getHeaders().getFirst("x-foo")).isEqualTo("bar");
+		assertThat(result.getHeaders()).doesNotContainKey("id");
+	}
+
+	@Test
+	public void headers() throws Exception {
+		ResponseEntity<String> result = rest.exchange(RequestEntity
+				.post(new URI("/headers")).contentType(MediaType.APPLICATION_JSON)
+				.body("[\"foo\",\"bar\"]"), String.class);
+		assertThat(result.getBody()).isEqualTo("[\"(FOO)\",\"(BAR)\"]");
+		assertThat(result.getHeaders().getFirst("foo")).isEqualTo("bar");
+		assertThat(result.getHeaders()).doesNotContainKey("id");
+	}
+
 	@Test
 	public void uppercaseSingleValue() throws Exception {
 		ResponseEntity<String> result = rest
@@ -406,6 +428,20 @@ public class RestApplicationTests {
 			return value -> "(" + value.trim().toUpperCase() + ")";
 		}
 
+		@Bean
+		public Function<Message<String>, Message<String>> messages() {
+			return value -> MessageBuilder
+					.withPayload("(" + value.getPayload().trim().toUpperCase() + ")")
+					.copyHeaders(value.getHeaders()).build();
+		}
+
+		@Bean
+		public Function<Flux<Message<String>>, Flux<Message<String>>> headers() {
+			return flux -> flux.map(value -> MessageBuilder
+					.withPayload("(" + value.getPayload().trim().toUpperCase() + ")")
+					.setHeader("foo", "bar").build());
+		}
+
 		@Bean
 		public Function<Flux<Foo>, Flux<Foo>> upFoos() {
 			return flux -> flux.log()