Don't use mock non-exist method assert_called_once

There isn't method assert_called_once of mock class
and hence does not really check anything. Let's use
assertEqual(1, mock.call_count) to check.

Change-Id: I2bdf62b775badc0adbe2c6f55bcc65cd13ac2d60
Session Documentation

Include some client documentation for how to work with a Session object.
This is generally high level with this expectation that a developer
could take the concepts found here and determine more from the module
documentation.

Change-Id: I5e44c1029ce160cb2798cfb8a535aa9f3311799a
Merge "Don't use mock non-exist method assert_called_once"
Merge "Session Documentation"