Add apache ExpiresByType equivalent functionality

[thermal7.myopenid.com]

Add apache ExpiresByType equivalent functionality. The closest thing in nginx at the moment is:

    # CSS and Javascript
    location ~* \.(?:css|js)$ {
      expires 1Y;
      access_log off;
      add_header Cache-Control "public";
    }

which is not ideal as the content type is a more reliable way to identify the type of a file. For example ​http://clientdependency.codeplex.com/ results in a URI of something like /DependcyHandler.axd?asfasdkfjashdfkashdfjkhasdk

I think this would best be done by something like

    # CSS and Javascript
    content_type text/css, text/javascript {
      expires 1Y;
      access_log off;
      add_header Cache-Control "public";
    }

or

    # CSS and Javascript
    for_header "content-type" > text/css, text/javascript {
      expires 1Y;
      access_log off;
      add_header Cache-Control "public";
    }

[Maxim Dounin <mdounin@…>]

In 4983f7d18fe36859a700cee06b162ae12aff5fa0/nginx:

Headers filter: variables support in expires (ticket #113).

[Maxim Dounin]

The expires directive now supports variables, and this allows to do something like:

map $sent_http_content_type $expires {
    default    off;
    image/jpeg 1w;
}

expires $expires;

Thanks to all involved.