Entity tag may be of length 2 as per RFC 2616, i.e. double quotes only.
Pointed out by Ruslan Ermilov.

It allows to disable generation of nginx's own entity tags, while
still handling ETags in cache properly.  This may be useful e.g.
if one want to serve static files from servers with different ETag
generation algorithms.

Notably this allows to clear ETag if one want to for some reason.

This includes handling of ETag headers (if present in a response) with
basic support for If-Match, If-None-Match conditionals in not modified
filter.

Note that the "r->headers_out.last_modified_time == -1" check in the not
modified filter is left as is intentionally.  It's to prevent handling
of If-* headers in case of proxy without cache (much like currently
done with If-Modified-Since).

This makes code more extendable.  The only functional change is when
If-Modified-Since and If-Unmodified-Since are specified together, the
case which is explicitly left undefined by RFC 2616.  The new behaviour
is to respect them both, which seems better.

This allows to use last modified time set in If-Range checks.  Code
simplified to improve readability.

If modification time isn't known, skip range processing and return full
entity body instead of just ignoring If-Range.  Ignoring If-Range isn't
safe as client will assume entity wasn't changed since time specified.