fixed logic to check status from our challenge

fixed logic to check status from our challenge

the old code had a problem and would interpret a challenge that
returned "pending" and then "invalid" as valid.

This code actually has another problem. The RFC defines:

"status (optional, string):  The status of this authorization.
 Possible values are: "pending", "valid", and "invalid".  If this
 field is missing, then the default value is "pending"."

So actually the correct way to implement this would be:

while [[ -z "${status}" ]] || [[ "${status}" = "pending" ]]; do

But without further checks this might lead to an endless loop. So this
is "good enough(tm)". ;)