switch to poAutoMakeBucket

Maxwell worried that allowing arbitrary nonstandard headers could cause
silent data loss if they were sent to a S3 endpoint that did not support
them. This avoids that problem, by only supporting the IA's
x-amz-auto-make-bucket header. If a server ignores that header, no data is
lost, although of course trying to PutObject into a bucket that does not
exist will fail.