ake ngx_mruby can read nginx build in variable and request variable.

```nginx

        # vars
        location /vars {
            # http://localhost/vars?version=ngx_mruby1.0
            set $foo "mruby";
            mruby_content_handler_code '
                r = Nginx::Request.new()
                r.content_type = "text/html"
                Nginx.rputs "#{r.var.arg_version}<br>"
                Nginx.rputs "#{r.var.host}<br>"
                Nginx.rputs "#{r.var.foo}<br>"
                Nginx.rputs "#{r.var.uri}<br>"
                Nginx.rputs "#{r.var.http_user_agent}<br>"
                Nginx.return Nginx::NGX_HTTP_OK
            ';
        }
```