Fix Assert.instanceOf exception message

Update the exception message used when Assert.instanceOf fails such
that it expects the provided message to end with '.'. This reverts
commit 5874383ef0 which caused the
implementation to be at odds with the JavaDoc and the previous
release.

The updated code also has the benefit of protecting against a null
message.

Issue: SPR-10269

spring-core/src/main/java/org/springframework/util/Assert.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2013 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. You may obtain a copy of
@@ -334,8 +334,9 @@ public abstract class Assert {
 	public static void isInstanceOf(Class type, Object obj, String message) {
 		notNull(type, "Type to check against must not be null");
 		if (!type.isInstance(obj)) {
-			throw new IllegalArgumentException(message +
-					". Object of class [" + (obj != null ? obj.getClass().getName() : "null") +
+			throw new IllegalArgumentException(
+					(StringUtils.hasLength(message) ? message + " " : "") +
+					"Object of class [" + (obj != null ? obj.getClass().getName() : "null") +
 					"] must be an instance of " + type);
 		}
 	}