xmerl_c14n:canon_name/3 fix.

We got somthing like ['http://www.w3.org/2000/09/xmldsig#',"Signature"] when
default namespace was used.

I observed this error when started my integration with centrify service.
This was reason why signature element was not stripped correctly.
example of xml:
https://gist.github.com/IgorKarymov/6776658