GH-1219: Fix header mapping for replies (@SendTo)

Resolves https://github.com/spring-projects/spring-amqp/issues/1219

The headers were mapped after message conversion.
This prevented using a `ContentTypeDelegatingMessageConverter` because
the content type was not set.

Add a header to control whether the user or converter gets to set the
content type property in the final message.

**cherry-pick to 2.2.x, 2.1.x, 1.7.x**

src/reference/asciidoc/amqp.adoc

@@ -2717,7 +2717,7 @@ public ReplyPostProcessor echoCustomHeader() {
 ----
 ====
 
-The `@SendTo` value is assumed as a reply `exchange` and `routingKey` pair that follws the `exchange/routingKey` pattern,
+The `@SendTo` value is assumed as a reply `exchange` and `routingKey` pair that follows the `exchange/routingKey` pattern,
 where one of those parts can be omitted.
 The valid values are as follows:
 
@@ -2800,6 +2800,28 @@ public String listen(Message in) {
 ----
 ====
 
+====== Reply ContentType
+
+If you are using a sophisticated message converter, such as the `ContentTypeDelegatingMessageConverter`, you can control the content type of the reply by returning a `spring-messaging` `Message<?>`:
+
+====
+[source, java]
+----
+@RabbitListener(queues = "q1")
+@SendTo("q2")
+public Message<String> listen(String in) {
+    ...
+    return MessageBuilder.withPayload(in.toUpperCase())
+            .setHeader(MessageHeaders.CONTENT_TYPE, "application/xml")
+            .build();
+}
+----
+====
+
+This content type will be passed in the `MessageProperties` to the converter.
+By default, for backwards compatibility, any content type property set by the converter will be overwritten by this value after conversion.
+If you wish to override that behavior, also set the `AmqpHeaders.CONTENT_TYPE_CONVERTER_WINS` to `true` and any value set by the converter will be retained.
+
 [[annotation-method-selection]]
 ====== Multi-method Listeners