Updated test case for the expiration setter (microseconds truncation)

The new test tries to make the fact that the datetime object given will
contain microseconds. It also tests for the fact that the microseconds
value is truncated after being set.
Updated side effects in the repository tool expiration setter

The expiration setter's docstring now mentions the fact that the
microseconds value in the given datetime object will be discarded if it
is non-zero.