b2: fix chunked streaming uploads

Streaming uploads are used by rclone rcat and rclone mount
--vfs-cache-mode off.

After the multipart chunker refactor the multipart chunked streaming
upload was accidentally mixing the first and the second parts up which
was causing corrupted uploads.

This was caused by a simple off by one error in the refactoring where
we went from 1 based part number counting to 0 based part number
counting.

Fixing this revealed that the metadata wasn't being re-read for the
copied object either.

This fixes both of those issues and adds an integration tests so it
won't happen again.

Fixes #7367

backend/b2/b2.go

@@ -1923,7 +1923,11 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 				return err
 			}
 			// NB Stream returns the buffer and token
-			return up.Stream(ctx, rw)
+			err = up.Stream(ctx, rw)
+			if err != nil {
+				return err
+			}
+			return o.decodeMetaDataFileInfo(up.info)
 		} else if err == io.EOF {
 			fs.Debugf(o, "File has %d bytes, which makes only one chunk. Using direct upload.", n)
 			defer o.fs.putRW(rw)