Assume standard storage class if not set in metadata (#5370)

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.

cmd/xl-v1-object_test.go

@@ -112,6 +112,8 @@ func TestXLDeleteObjectBasic(t *testing.T) {
 }
 
 func TestXLDeleteObjectDiskNotFound(t *testing.T) {
+	// Reset global storage class flags
+	resetGlobalStorageEnvs()
 	// Create an instance of xl backend.
 	obj, fsDirs, err := prepareXL16()
 	if err != nil {