Refactor PropertyResolver impl for consistency

PropertySourcesPropertyResolver#containsProperty now
calls #containsProperty on each underlying PropertySource instead of
calling #getProperty and checking for null.

Issue: SPR-9529

Author: cbeams

spring-core/src/main/java/org/springframework/core/env/PropertySourcesPropertyResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2011 the original author or authors.
+ * Copyright 2002-2012 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.
@@ -45,7 +45,7 @@ public PropertySourcesPropertyResolver(PropertySources propertySources) {
 
 	public boolean containsProperty(String key) {
 		for (PropertySource<?> propertySource : this.propertySources) {
-			if (propertySource.getProperty(key) != null) {
+			if (propertySource.containsProperty(key)) {
 				return true;
 			}
 		}