* Fixes #929 : error in Client.activate_session()

Fixes the error that a call to Client.activate_session() without arguments overwrites a user certificate previously set with Client.load_client_certificate(user_cert_file).

* Do not override already set user certificate

If you set the user certificate with Client.load_client_certificate() and then activate the session with Client.activate_session() without passing the certificate as an argument, the previously set certificate is discarded.