Reference to the XML spec

All,

The XACML 3.0 references the XML specification. In CD-1 it is the fourth 
edition of XML, but there is a fifth edition which has been accepted as 
a recommendation by the W3C. I think we should update the reference.

There is not really a difference since we use this reference only to 
define what is whitespace for some of the string functions, but I think 
it looks better. (XACML 2.0 does not have this reference at all.)

Best regards,
Erik