GH-730: Fix NPE in the MessageProperties

Fixes https://github.com/spring-projects/spring-amqp/issues/730

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

@@ -568,6 +568,14 @@ public class MessageProperties implements Serializable {
 		if (!Arrays.equals(this.correlationId, other.correlationId)) {
 			return false;
 		}
+		if (this.correlationIdString == null) {
+			if (other.correlationIdString != null) {
+				return false;
+			}
+		}
+		else if (!this.correlationIdString.equals(other.correlationIdString)) {
+			return false;
+		}
 		if (this.deliveryMode != other.deliveryMode) {
 			return false;
 		}

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

@@ -17,6 +17,7 @@
 package org.springframework.amqp.core;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
 
 import org.junit.Test;
 
@@ -41,4 +42,11 @@ public class MessagePropertiesTests {
 		assertEquals(null, properties.getReplyToAddress());
 	}
 
+	@Test
+	public void tesNoNullPointerInEquals() {
+		MessageProperties mp = new MessageProperties();
+		MessageProperties mp2 = new MessageProperties();
+		assertTrue(mp.equals(mp2));
+	}
+
 }