GH-918: Handle null correlationId

Fixes https://github.com/spring-projects/spring-amqp/issues/918

* Add unit test and `@autor`

**Cherry-pick to 2.0.x & 1.7.x**

spring-amqp/src/main/java/org/springframework/amqp/core/MessageProperties.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@ import java.util.Map;
  * @author Gary Russell
  * @author Dmitry Chernyshov
  * @author Artem Bilan
+ * @author Csaba Soti
  */
 public class MessageProperties implements Serializable {
 
@@ -525,7 +526,7 @@ public class MessageProperties implements Serializable {
 		result = prime * result + ((this.contentEncoding == null) ? 0 : this.contentEncoding.hashCode());
 		result = prime * result + (int) (this.contentLength ^ (this.contentLength >>> 32));
 		result = prime * result + ((this.contentType == null) ? 0 : this.contentType.hashCode());
-		result = prime * result + this.correlationId.hashCode();
+		result = prime * result + ((this.correlationId == null) ? 0 : this.correlationId.hashCode());
 		result = prime * result + ((this.deliveryMode == null) ? 0 : this.deliveryMode.hashCode());
 		result = prime * result + (int) (this.deliveryTag ^ (this.deliveryTag >>> 32));
 		result = prime * result + ((this.expiration == null) ? 0 : this.expiration.hashCode());

spring-amqp/src/test/java/org/springframework/amqp/core/MessagePropertiesTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,6 +20,9 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
+import java.util.HashSet;
+import java.util.Set;
+
 import org.junit.Test;
 
 
@@ -27,12 +30,12 @@ import org.junit.Test;
  * @author Dave Syer
  * @author Artem Yakshin
  * @author Artem Bilan
+ * @author Csaba Soti
  *
  */
 public class MessagePropertiesTests {
 
 
-
 	@Test
 	public void testReplyTo() throws Exception {
 		MessageProperties properties = new MessageProperties();
@@ -71,4 +74,11 @@ public class MessagePropertiesTests {
 		assertTrue(mp.equals(mp2));
 	}
 
+	@Test
+	public void tesNoNullPointerInHashCode() {
+		Set<MessageProperties> messageList = new HashSet<>();
+		messageList.add(new MessageProperties());
+		assertEquals(1, messageList.size());
+	}
+
 }