Made CBLRemoteRequest JSON error detection work with CouchDB-type server
Made CBLRemoteRequest JSON error detection work with CouchDB-type server

Fixes a minor bug in 6f3c6fcf -- the workaround for CouchDB not returning
JSON as application/json didn't have the right string constant to
detect the "text/plain" type it returns.

For #1245

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