Guard NPE in HeartbeatMonitor

fixes gh-129

spring-cloud-commons/src/main/java/org/springframework/cloud/client/discovery/event/HeartbeatMonitor.java

@@ -34,7 +34,7 @@ public class HeartbeatMonitor {
 	 */
 	public boolean update(Object value) {
 		Object last = this.latestHeartbeat.get();
-		if (!value.equals(last)) {
+		if (value != null && !value.equals(last)) {
 			return this.latestHeartbeat.compareAndSet(last, value);
 		}
 		return false;

spring-cloud-commons/src/test/java/org/springframework/cloud/client/discovery/event/HeartbeatMonitorTests.java

@@ -40,4 +40,15 @@ public class HeartbeatMonitorTests {
 		assertTrue(this.monitor.update("bar"));
 	}
 
+	@Test
+	public void nullInitialValue() {
+		assertFalse(this.monitor.update(null));
+	}
+
+	@Test
+	public void nullSecondValue() {
+		assertTrue(this.monitor.update("foo"));
+		assertFalse(this.monitor.update(null));
+	}
+
 }