SystemPropertyModule is an JXPath based InputModule implementation that
provides access to system properties.
Available system properties are defined by Java's System.getProperties().
JXPath allows to apply XPath functions to system properties.
If there is a security manager, its checkPropertiesAccess
method is called with no arguments. This may result in a security exception
which is wrapped into a configuration exception and re-thrown.
Returns the object which should be used as JXPath context.
Descendants should override this method to return a specific object
that is requried by the implementing class.
Examples are: request, session and application context objects.