Honor headers in AbstractMessageSendingTemplate.convertAndSend() variant

Prior to this commit, the following method in AbstractMessageSendingTemplate
simply ignored the supplied headers map.

convertAndSend(Object, Map<String, Object>, MessagePostProcessor)

Closes gh-36120

spring-messaging/src/main/java/org/springframework/messaging/core/AbstractMessageSendingTemplate.java

@@ -148,7 +148,7 @@ public abstract class AbstractMessageSendingTemplate<D> implements MessageSendin
 	public void convertAndSend(Object payload, @Nullable Map<String, Object> headers,
 			@Nullable MessagePostProcessor postProcessor) throws MessagingException {
 
-		convertAndSend(getRequiredDefaultDestination(), payload, null, postProcessor);
+		convertAndSend(getRequiredDefaultDestination(), payload, headers, postProcessor);
 	}
 
 	@Override

spring-messaging/src/test/java/org/springframework/messaging/core/GenericMessagingTemplateTests.java

@@ -17,6 +17,7 @@
 package org.springframework.messaging.core;
 
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicReference;
@@ -50,6 +51,7 @@ import static org.mockito.Mockito.verify;
  *
  * @author Rossen Stoyanchev
  * @author Gary Russell
+ * @author Sam Brannen
  */
 class GenericMessagingTemplateTests {
 
@@ -248,6 +250,20 @@ class GenericMessagingTemplateTests {
 		assertThat(accessor.isMutable()).isFalse();
 	}
 
+	@Test
+	void convertAndSendWithDefaultDestinationAndSimpMessageHeaders() {
+		Map<String, Object> headers = Map.of("foo", "bar");
+		Object payload = "Hello";
+		TestMessagePostProcessor postProcessor = new TestMessagePostProcessor();
+
+		this.template.convertAndSend(payload, headers, postProcessor);
+		List<Message<byte[]>> messages = this.messageChannel.getMessages();
+		Message<byte[]> message = messages.get(0);
+
+		assertThat(postProcessor.getMessage().getPayload()).isEqualTo(payload);
+		assertThat(message.getHeaders()).containsKeys("foo");
+	}
+
 
 	private class TestDestinationResolver implements DestinationResolver<MessageChannel> {