If cache was bypassed with proxy_cache_bypass, cache-controlling headers
(Cache-Control, Expires) wasn't considered and response was cached even
if it was actually non-cacheable.

Patch by John Ferlito.

Patch by Peter Smit.

"start" parameter value.

See here for report:
http://mailman.nginx.org/pipermail/nginx-ru/2011-September/043288.html

Configuration with duplicate upstream blocks defined after first use, i.e.
like

    server {
        ...
        location / {
            proxy_pass http://backend;
        }
    }

    upstream backend { ... }
    upstream backend { ... }

now correctly results in "duplicate upstream" error.

Additionally, upstream blocks defined after first use now handle various
server directive parameters ("weight", "max_fails", etc.).  Previously
configuration like

    server {
        ...
        location / {
            proxy_pass http://backend;
        }
    }

    upstream backend {
        server 127.0.0.1 max_fails=5;
    }

incorrectly resulted in "invalid parameter "max_fails=5"" error.

For normal cached responses ngx_http_cache_send() sends last buffer and then
request finalized via ngx_http_finalize_request() call, i.e. everything is
ok.

But for stale responses (i.e. when upstream died, but we have something in
cache) the same ngx_http_cache_send() sends last buffer, but then in
ngx_http_upstream_finalize_request() another last buffer is send.  This
causes duplicate final chunk to appear if chunked encoding is used (and
resulting problems with keepalive connections and so on).

Fix this by not sending in ngx_http_upstream_finalize_request()
another last buffer if we know response was from cache.

Revert wrong fix for empty responses introduced in 0.8.31 and apply new one,
rewritten to match things done by static module as close as possible.