Added a method for extracting a location strategy from a FileLocator.

In a FileLocator the default location strategy can be overridden. The new
method always returns a valid strategy, no matter whether the locator
defines one or not.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1527227 13f79535-47bb-0310-9956-ffa450edef68