netstorage: fix missing MD5 hash after uploading a file

The object returned from Put and Update had an empty MD5 checksum
while a fresh listing would report the MD5 the server computed for
the upload. This broke wrappers which compare exact hashes, such as
the hasher backend's fingerprint and the VFS cache. Stat the file
after upload to pick up the server computed MD5.

backend/netstorage/netstorage.go

@@ -1050,14 +1050,15 @@ func (o *Object) netStorageUploadRequest(ctx context.Context, in io.Reader, src
 
 	// Invalidate stat cache
 	o.fs.deleteStatCache(URL)
-	if o.size == -1 {
+	// Stat the uploaded file to get the md5 the server computed and
+	// the size for streaming uploads.
 	files, err := o.fs.netStorageStatRequest(ctx, URL, false)
 	if err != nil {
 		return nil
 	}
 	if files != nil {
 		o.size = files[0].Size
-		}
+		o.md5sum = files[0].Md5
 	}
 	return nil
 }