Add support for rendering single value in response

If the request is a single value, we already know, so if we also
know that the function returned a single-valued type, then we can
render the single value, instead of an array with a single item.

See gh-36

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

@@ -283,6 +283,25 @@ public class RestApplicationTests {
 				.isEqualTo("[{\"value\":\"FOO\"}]");
 	}
 
+	@Test
+	public void bareUppercaseFoos() throws Exception {
+		ResponseEntity<String> result = rest.exchange(RequestEntity
+				.post(new URI("/bareUpFoos")).contentType(MediaType.APPLICATION_JSON)
+				.body("[{\"value\":\"foo\"},{\"value\":\"bar\"}]"), String.class);
+		assertThat(result.getBody())
+				.isEqualTo("[{\"value\":\"FOO\"},{\"value\":\"BAR\"}]");
+	}
+
+	@Test
+	public void bareUppercaseFoo() throws Exception {
+		// Single Foo can be parsed and returns a single value if the function is defined that way
+		ResponseEntity<String> result = rest.exchange(RequestEntity
+				.post(new URI("/bareUpFoos")).contentType(MediaType.APPLICATION_JSON)
+				.body("{\"value\":\"foo\"}"), String.class);
+		assertThat(result.getBody())
+				.isEqualTo("{\"value\":\"FOO\"}");
+	}
+
 	@Test
 	public void bareUppercase() throws Exception {
 		ResponseEntity<String> result = rest.exchange(RequestEntity
@@ -382,6 +401,11 @@ public class RestApplicationTests {
 					.map(value -> new Foo(value.getValue().trim().toUpperCase()));
 		}
 
+		@Bean
+		public Function<Foo, Foo> bareUpFoos() {
+			return value -> new Foo(value.getValue().trim().toUpperCase());
+		}
+
 		@Bean
 		public Function<Flux<Integer>, Flux<String>> wrap() {
 			return flux -> flux.log().map(value -> ".." + value + "..");