Add more subtle content negotiation in web layer

So that single Strings can be POSTed without JSON conversion.
There's still some work to do to support single POJOs in JSON, and
to reach parity with the WebFlux reactive type handlers, but it's
now closer to what we had before we moved the String conversion
out of the function layer.

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

@@ -46,6 +46,7 @@ public class SampleApplicationTests {
 
 	@Test
 	public void uppercase() {
+		// TODO: make this work with a JSON stream as well (like in WebFlux)
 		assertThat(new TestRestTemplate().postForObject(
 				"http://localhost:" + port + "/uppercase", "[{\"value\":\"foo\"}]",
 				String.class)).isEqualTo("[{\"value\":\"FOO\"}]");