saveBack doesn't update eTag

[josephguillaume]

As saveBack doesn't update eTag after the PUT, subsequent attempts to save fail with 412 Precondition Failed as the eTag sent is then out-dated

https://github.com/solid/source-pane/blob/14fb2fb70c136ab89e1e28d8935a6d6925e1eb7d/sourcePane.js#L195-L200

This seems like a trivial fix, using the same line as in refresh/GET.

https://github.com/solid/source-pane/blob/14fb2fb70c136ab89e1e28d8935a6d6925e1eb7d/sourcePane.js#L247

However, this will only work if the server supports strong etags in the first place.
CSS currently does not sent an updated eTag on PUT: CommunitySolidServer/CommunitySolidServer#632

[josephguillaume]

The discussion on the community solid server issue has now indicated that an ETag will not be provided on PUT. My understanding is that saveBack should therefore do a GET or HEAD to fetch a new ETag after a successful PUT (or can websockets be used to be notified of the change?)