wfe: always render authz challenges as an array (#258)
wfe: always render authz challenges as an array (#258)

From https://tools.ietf.org/html/rfc8555#section-7.1.4

  (required, array of objects)

This commit fixes the case where Pebble renders "challenge: null"
when e.g. the authorization has been deactivated.
letsencrypt/pebble,letsencrypt/pebble,letsencrypt/pebble