Condition a chunk with a tag

[k0pernicus]

A good observation from slamb in the Reddit post announcement is:

It looks like your chunk GET requests are not conditioned on an etag, or even on a Last-Modified header.
So if the server doesn't serve the same file every time (such as if the file were replaced on the server
halfway through), you'll blindly put together parts of different files without any warning.
This checking shouldn't be hard to add; I think it'd be better to do it and warn the user in this case.

I agree with this comment, and I think we should warn the user as a first case, and implement a better solution for a next release.

[Jean-Serge]

Yes but it seems this issue depends on the #32 one. If we do not implement the behaviour for interrupting download, it has no sense to use e-tag.

What do you think ?