Add Support for Weak ETags

[Piotr Sikora]

Hey Maxim,

> The sub and ssi remove Last-Modified and all etags by default.  If
> needed though, they can be configured to preserve Last-Modified
> (and weak etags with a patch) with ssi_last_modified directive and
> friends:
>
> http://nginx.org/r/ssi_last_modified
>
> (...)
>
> Much like with ssi, this is the default.  I don't think we should
> remove weak etags if we are explicitly configured to preserve
> Last-Modified though (http://nginx.org/r/xslt_last_modified).

Sorry, I've completely missed the presence of _last_modified directives.

You're right, your changes look better than just gzip/gunzip.

Best regards,
Piotr Sikora