Allow sending binary messages with STOMP

After this change the StompSubProtocolHandler sends STOMP frames
with content-type=octet-stream as a binary WebSocket message.

Issue: SPR-12475

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

@@ -52,6 +52,8 @@
 import org.springframework.messaging.support.MessageHeaderAccessor;
 import org.springframework.messaging.support.MessageHeaderInitializer;
 import org.springframework.util.Assert;
+import org.springframework.util.MimeType;
+import org.springframework.util.MimeTypeUtils;
 import org.springframework.web.socket.BinaryMessage;
 import org.springframework.web.socket.CloseStatus;
 import org.springframework.web.socket.TextMessage;
@@ -355,7 +357,12 @@ else if (StompCommand.CONNECTED.equals(command)) {
 		}
 		try {
 			byte[] bytes = this.stompEncoder.encode(stompAccessor.getMessageHeaders(), (byte[]) message.getPayload());
-			session.sendMessage(new TextMessage(bytes));
+			if (MimeTypeUtils.APPLICATION_OCTET_STREAM.isCompatibleWith(stompAccessor.getContentType())) {
+				session.sendMessage(new BinaryMessage(bytes));
+			}
+			else {
+				session.sendMessage(new TextMessage(bytes));
+			}
 		}
 		catch (SessionLimitExceededException ex) {
 			// Bad session, just get out

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/AbstractSockJsSession.java

@@ -30,7 +30,6 @@
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
-
 import org.springframework.core.NestedCheckedException;
 import org.springframework.util.Assert;
 import org.springframework.web.socket.CloseStatus;
@@ -158,9 +157,7 @@ public Map<String, Object> getAttributes() {
 
 	public final void sendMessage(WebSocketMessage<?> message) throws IOException {
 		Assert.state(!isClosed(), "Cannot send a message when session is closed");
-		if (!(message instanceof TextMessage)) {
-			throw new IllegalArgumentException("Expected text message: " + message);
-		}
+		Assert.isInstanceOf(TextMessage.class, message, "SockJS supports text messages only: " + message);
 		sendMessageInternal(((TextMessage) message).getPayload());
 	}