Expired Authorizations/Challenges are still pending

[jvanasco]

If an Authorization has expired, loading it will show a "pending" status.
If a Challenge belongs to an Authorization which has expired, loading it will show a "pending" status.

Triggering the expired Challenge will result in an error json payload, as expected, such as:

 {u'status': 400, u'type': u'urn:ietf:params:acme:error:malformed', u'detail': u'Authorization expired 2020-02-28T20:25:02Z'}

Attempting to load the Authorization or Challenge after receiving this payload will still return a "pending" status.