Use `described_class` in all specs

This is a convenient shorthand for RSpec tests, so we should use it in
the interest of keeping tests concise and DRY.

Change-Id: I6cbb510b38bafe24fb249355338dcbffe64083e4
Reviewed-on: https://gerrit.causes.com/23144
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>