cleanup_for_epub3 prevents reproducible builds

[slonopotamus]

Currently, it is impossible to create two byte-to-byte identical EPUB files via gepub because it unconditionally sets modified time to current time when user calls generate_epub_stream or generate_epub.

Please, add an option to disable this behavior, so that gepub user can explicitly set modified date to a fixed timestamp. Alternative way: only update modified date in cleanup_for_epub3 if it wasn't set explicitly.

More info about reproducible builds concept is available on https://reproducible-builds.org/

[slonopotamus]

After a bit more thinking, possibly what you did is not as good. Now, last modified will not update if you load existing epub, modify it and save back. Right? Seems like, we actually need an option to say whether to update last modified date or not.

[slonopotamus]

@skoji what do you think?

[skoji]

Now, last modified will not update if you load existing epub, modify it and save back. Right?

Yes, that’s right. Original behavior is intended to prevent that case.

But the original behavior also made book.lastmodified= meaningless.

[skoji]

OK I'm going to change on parsed EPUB:

Update lastmodified on save even if parsed lastmodified exists, unless client explicitly sets lastmodified.