Fix unit test for expiration date without date but with default

The expiration date was explicitly given, so it was not testing what it
should. Moreover, "link_defaultExpDays" needs to be mapped when testing
the default value (even if it will just fallback to the value of
"shareapi_expire_after_n_days").

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>