Ensure StompEncoder never writes content-length twice

Issue: SPR-11984

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompEncoder.java

@@ -112,8 +112,12 @@ public final class StompEncoder  {
 
 		for (Entry<String, List<String>> entry : nativeHeaders.entrySet()) {
 			byte[] key = encodeHeaderString(entry.getKey(), shouldEscape);
+			if (command.requiresContentLength() && "content-length".equals(entry.getKey())) {
+				continue;
+			}
 			List<String> values = entry.getValue();
-			if (StompHeaderAccessor.STOMP_PASSCODE_HEADER.equals(entry.getKey())) {
+			if (StompCommand.CONNECT.equals(command) &&
+					StompHeaderAccessor.STOMP_PASSCODE_HEADER.equals(entry.getKey())) {
 				values = Arrays.asList(StompHeaderAccessor.getPasscode(headers));
 			}
 			for (String value : values) {

spring-messaging/src/test/java/org/springframework/messaging/simp/stomp/StompCodecTests.java

@@ -290,6 +290,17 @@ public class StompCodecTests {
 				new Reactor11StompCodec().encoder().apply(frame).asString());
 	}
 
+	@Test
+	public void encodeFrameWithContentLengthPresent() {
+		StompHeaderAccessor headers = StompHeaderAccessor.create(StompCommand.SEND);
+		headers.setContentLength(12);
+
+		Message<byte[]> frame = MessageBuilder.createMessage("Message body".getBytes(), headers.getMessageHeaders());
+
+		assertEquals("SEND\ncontent-length:12\n\nMessage body\0",
+				new Reactor11StompCodec().encoder().apply(frame).asString());
+	}
+
 	private void assertIncompleteDecode(String partialFrame) {
 		Buffer buffer = Buffer.wrap(partialFrame);
 		assertNull(decode(buffer));