Make containsDescendantOf a default method

Change `ConfigurationPropertySource.containsDescendantOf` to have a
default implementation that return `UNKNOWN`.

Fixes gh-12539

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/ConfigurationPropertySource.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -50,7 +50,10 @@ public interface ConfigurationPropertySource {
 	 * @param name the name to check
 	 * @return if the source contains any descendants
 	 */
-	ConfigurationPropertyState containsDescendantOf(ConfigurationPropertyName name);
+	default ConfigurationPropertyState containsDescendantOf(
+			ConfigurationPropertyName name) {
+		return ConfigurationPropertyState.UNKNOWN;
+	}
 
 	/**
 	 * Return a filtered variant of this source, containing only names that match the