Merge of r4651: proxy_cookie_* fix.

Merge of r4651: proxy_cookie_* fix.

Fixed returned value handling from the cookie rewrite handler.

If the "proxy_cookie_domain" or "proxy_cookie_path" directive is used and there
are no matches in Set-Cookie header then ngx_http_proxy_rewrite_cookie() returns
NGX_DECLINED to indicate that the header was not rewritten. Returning this value
further from the upstream headers copy handler resulted in 500 error response.

See here for report:
http://mailman.nginx.org/pipermail/nginx/2012-May/033858.html