Wrong idempotence check setting session attributes

[dvarrazzo]

In [9]: cnn.autocommit = False
[23759] conn_set_session: autocommit 0, isolevel 5, readonly 2, deferrable 2

In [10]: cnn.readonly = True
[23759] conn_set_session: autocommit 0, isolevel 5, readonly 1, deferrable 2

In [11]: cnn.autocommit = True
[23759] conn_set_session: autocommit 1, isolevel 5, readonly 1, deferrable 2

In [12]: cnn.readonly = True
[23759] conn_set_session: autocommit 1, isolevel 5, readonly 1, deferrable 2

The operation in [12] would be idempotent: the code checks that the
new value is the same of the previous one and bails out early. But
checking the internal state for idempotence when in autocommit is
wrong: the only state that matters is what's in the server session.

[dvarrazzo]

Reference: https://www.postgresql.org/message-id/CA%2Bmi_8YAzSm%3Dpb4XqUZvEvUrtL30p1f1QyBViJQfe7xUcBBpjA%40mail.gmail.com