Fix 231-cors.t test suite (again)

That's interesting issue: couch_passwords:hash_admin_password accepts
password as binary string, but list one had been passed instead. This
causes crush with function_clause reason. Ok, but this crush left
hidden for R15/R16 - only R14 shows stack trace in output and alerts
that's something wrong. To be honest, *sometimes* it's also possible
to reproduce this test suite crush with modern Erlang releases,
but it will be about Bad Plan: planned 27 test, but run only 26.
Nothing specific.

So, silent crush prevented other tests to be run and also counted by
the plan. Now this is fixed.