Allow URI attribute of Reference to be absent (#102)

The schema and standard allow for the absence of the URI attribute of
the <ds:Reference> element. For example the signed XML files that
oath-toolkit produces are missing this attribute.

Author: arthurdejong

signxml/__init__.py

@@ -223,7 +223,7 @@ def _c14n(self, nodes, algorithm, inclusive_ns_prefixes=None):
 
     def _resolve_reference(self, doc_root, reference, uri_resolver=None):
         uri = reference.get("URI")
-        if uri == "":
+        if not uri:
             return doc_root
         elif uri.startswith("#xpointer("):
             raise InvalidInput("XPointer references are not supported")