Keep add_sign backward compatibility

Author: pitbulk

src/onelogin/saml2/utils.py

@@ -742,7 +742,11 @@ def add_sign(xml, key, cert, debug=False, sign_algorithm=OneLogin_Saml2_Constant
             issuer = issuer[0]
             issuer.addnext(signature)
         else:
-            elem.insert(0, signature)
+            entity_descriptor = OneLogin_Saml2_XML.query(elem, '//md:EntityDescriptor')
+            if len(entity_descriptor) > 0:
+                elem.insert(0, signature)
+            else:
+                elem[0].insert(0, signature)
 
         elem_id = elem.get('ID', None)
         if elem_id: