Unsupported Binding exception in SLO

There's the possibility that an IdP doesn't support the SLO, added an exception handling as a workaround for this.
Now we have in the log something like:

````
Unsupported binding: None (https://172.17.0.1:10000/Saml2IDP/metadata)
Error Handled - SLO - unsupported binding by IDP: None
Traceback (most recent call last):
  File "~/DEV/IdentityPython/env/lib/python3.8/site-packages/djangosaml2/views.py", line 601, in get
    result = client.global_logout(subject_id)
  File "~/DEV/IdentityPython/env/lib/python3.8/site-packages/saml2/client.py", line 210, in global_logout
    return self.do_logout(
  File "~/DEV/IdentityPython/env/lib/python3.8/site-packages/djangosaml2/overrides.py", line 25, in do_logout
    return super().do_logout(*args, **kwargs)
  File "~/DEV/IdentityPython/env/lib/python3.8/site-packages/saml2/client.py", line 259, in do_logout
    bindings_slo_supported = self.metadata.single_logout_service(
  File "~/DEV/IdentityPython/env/lib/python3.8/site-packages/saml2/mdstore.py", line 1236, in single_logout_service
    return self.service(entity_id, "%s_descriptor" % typ,
  File "~/DEV/IdentityPython/env/lib/python3.8/site-packages/saml2/mdstore.py", line 1147, in service
    raise UnsupportedBinding(binding)
saml2.s_utils.UnsupportedBinding: None
````

Author: peppelinux

djangosaml2/views.py

@@ -596,18 +596,25 @@ def get(self, request, *args, **kwargs):
             logger.warning(
                 'The session does not contain the subject id for user %s', request.user)
 
+        _error = None
         try:
             result = client.global_logout(subject_id)
         except LogoutError as exp:
             logger.exception(
                 'Error Handled - SLO not supported by IDP: {}'.format(exp))
-            auth.logout(request)
-            state.sync()
-            return self.handle_unsupported_slo_exception(request, exp)
+            _error = exp
+        except UnsupportedBinding as exp:
+            logger.exception(
+                'Error Handled - SLO - unsupported binding by IDP: {}'.format(exp))
+            _error = exp
 
         auth.logout(request)
         state.sync()
 
+        if _error:
+            return self.handle_unsupported_slo_exception(request, _error)
+
+
         if not result:
             logger.error(
                 "Looks like the user %s is not logged in any IdP/AA", subject_id)