Work around Cloudant 403 error when POSTing to _changes
Work around Cloudant 403 error when POSTing to _changes

We had only seen 401 and 405 errors before, but now a 403 was reported,
so I've added it. (I also refactored the code a little for readability.)

Fixes #1267

amco/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios