Fix failing tests

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

@@ -40,6 +40,7 @@ import org.mockito.MockitoAnnotations;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageDeliveryException;
 import org.springframework.messaging.converter.MessageConversionException;
+import org.springframework.messaging.converter.StringMessageConverter;
 import org.springframework.messaging.simp.stomp.StompSession.Receiptable;
 import org.springframework.messaging.simp.stomp.StompSession.Subscription;
 import org.springframework.messaging.support.MessageBuilder;
@@ -85,6 +86,7 @@ public class DefaultStompSessionTests {
 		this.sessionHandler = mock(StompSessionHandler.class);
 		this.connectHeaders = new StompHeaders();
 		this.session = new DefaultStompSession(this.sessionHandler, this.connectHeaders);
+		this.session.setMessageConverter(new StringMessageConverter());
 
 		SettableListenableFuture<Void> future = new SettableListenableFuture<>();
 		future.set(null);