Make sure that userinfos always contain the issuer

Closes #42

feudal/backend/models/users.py

@@ -389,6 +389,10 @@ class User(AbstractUser):
         if 'given_name' in userinfo:
             self.first_name = userinfo['given_name']
 
+        # make sure the userinfo always contains the issuer uri
+        if 'iss' not in userinfo:
+            userinfo['iss'] = self.idp.issuer_uri
+
         self.userinfo = userinfo
         self.save()