GH-730: Fix NPE in the MessageProperties

Fixes #730

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

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -590,9 +590,16 @@ else if (!this.contentEncoding.equals(other.contentEncoding)) {
 		else if (!this.contentType.equals(other.contentType)) {
 			return false;
 		}
-		if (!this.correlationId.equals(other.correlationId)) {
+
+		if (this.correlationId == null) {
+			if (other.correlationId != null) {
+				return false;
+			}
+		}
+		else if (!this.correlationId.equals(other.correlationId)) {
 			return false;
 		}
+
 		if (this.deliveryMode != other.deliveryMode) {
 			return false;
 		}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -26,10 +26,13 @@
 /**
  * @author Dave Syer
  * @author Artem Yakshin
+ * @author Artem Bilan
  *
  */
 public class MessagePropertiesTests {
 
+
+
 	@Test
 	public void testReplyTo() throws Exception {
 		MessageProperties properties = new MessageProperties();
@@ -61,4 +64,11 @@ public void testContentLengthSet() {
 		assertTrue(properties.isContentLengthSet());
 	}
 
+	@Test
+	public void tesNoNullPointerInEquals() {
+		MessageProperties mp = new MessageProperties();
+		MessageProperties mp2 = new MessageProperties();
+		assertTrue(mp.equals(mp2));
+	}
+
 }