when locking a file set If-Match header to ensure etag is correct

when locking a file set If-Match header to ensure etag is correct

the file being locked may have been changed sinc the client synced it

so the server side file may have a modified etag. In such cases we do
not want to lock the file and would rather sync the nex server changes
before being able to lock the file

that would ensure that on client side the file being locked is matching
teh state of teh file on server side and would prevent inadvertently
overriding server changes

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>