If STANDARD storage class is set before starting up Minio server,
but x-amz-storage-class metadata field is not set in a PutObject
request, Minio server defaults to N/2 data and N/2 parity disks.
This PR changes the behaviour to use data and parity disks set in
STANDARD storage class, even if x-amz-storage-class metadata
field is not present in PutObject requests.