Add Support for Weak ETags

[Piotr Sikora]

Hey Maxim,

> I don't think that limiting the scope to gzip and gunzip is
> correct either.  From cache validation point of view, weak etags
> are mostly identical to Last-Modified, and removing etags
> completely should mostly follow ngx_http_clear_last_modified().

I strongly disagree. Modifying content (via addition/sub/ssi modules)
can change enough to consider two pages completely different (at least
as a general rule), so I don't think that they should retain any
ETags.

I've got mixed feelings regarding xslt module. While I don't disagree
as much, I think it's still safer to remove weak ETags there as well.

Best regards,
Piotr Sikora