The XACML 1.0 specification does not explicitly 
define the way to convert each of the selected node set 
into a string representation so that the constructor
functions can be applied.

In these tests, it is assumed that:
1) An XML attribute node is converted into 
the string representation of the attribute value.
2) An XML text node is converted into 
the string representation of the text node content.

In the DOM API, such a string representation can 
be obtained by org.w3c.dom.Node#getNodeValue.

In XACML 1.1, this assumption will be true.