ID attribute fix

Currently, the XML schema defines a ID type just like string or integer that should be used when declaring an ID attribute within a node in xml... that's right, naming an attribute "ID" is not enough anymore.

The xml parser from previous java versions (earlier than 7u25) was very flexible about this constraint and did not require the ID type on "ID" attributes, but this is not the case with the newest java version. 

With this fix, the isValid() method will traverse the xml document searching for attributes called "ID" and tag them as valid ID xml type, this will avoid an exception during the xmlSignature.validate(ctx) call.