Apply server configuration as soon as host is known. Previously, this was done only after the whole request header was parsed, and if an error occurred earlier then the request was processed in the default server (or server chosen by SNI), while r->headers_in.server might be set to the value from the Host: header or host from request line. r->headers_in.server is in turn used for $host variable and in HTTP redirects if "server_name_in_redirect" is disabled. Without the change, configurations that rely on this during error handling are potentially unsafe if SNI is used. This change also allows to use server specific settings of "underscores_in_headers", "ignore_invalid_headers", and "large_client_header_buffers" directives for HTTP requests and HTTPS requests without SNI.

scgi_param  REQUEST_METHOD     $request_method;
scgi_param  REQUEST_URI        $request_uri;
scgi_param  QUERY_STRING       $query_string;
scgi_param  CONTENT_TYPE       $content_type;

scgi_param  DOCUMENT_URI       $document_uri;
scgi_param  DOCUMENT_ROOT      $document_root;
scgi_param  SCGI               1;
scgi_param  SERVER_PROTOCOL    $server_protocol;
scgi_param  HTTPS              $https if_not_empty;

scgi_param  REMOTE_ADDR        $remote_addr;
scgi_param  REMOTE_PORT        $remote_port;
scgi_param  SERVER_PORT        $server_port;
scgi_param  SERVER_NAME        $server_name;