Support HTTP GET of single value value Function

E.g. a Function<Long, Foo> can be used to fetch a single entity of
type Foo with a long ID., via /{function}/{id}

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

@@ -150,6 +150,16 @@ public class RestApplicationTests {
 				.isEqualTo("[FOO][BAR]");
 	}
 
+	@Test
+	public void uppercaseGet() {
+		assertThat(rest.getForObject("/uppercase/foo", String.class)).isEqualTo("[FOO]");
+	}
+
+	@Test
+	public void convertGet() {
+		assertThat(rest.getForObject("/wrap/123", String.class)).isEqualTo("..123..");
+	}
+
 	@Test
 	public void uppercaseJsonArray() throws Exception {
 		assertThat(rest.exchange(
@@ -193,6 +203,11 @@ public class RestApplicationTests {
 					.map(value -> "[" + value.trim().toUpperCase() + "]");
 		}
 
+		@Bean
+		public Function<Flux<Integer>, Flux<String>> wrap() {
+			return flux -> flux.log().map(value -> ".." + value + "..");
+		}
+
 		@Bean
 		public Function<Flux<HashMap<String, String>>, Flux<Map<String, String>>> maps() {
 			return flux -> flux.map(value -> {