Fixed incorrect response line on "return 203".

Reported by Weibin Yao,
http://mailman.nginx.org/pipermail/nginx-devel/2013-April/003607.html.

src/http/ngx_http_header_filter_module.c

diff --git a/src/http/ngx_http_header_filter_module.c b/src/http/ngx_http_header_filter_module.c
index 707a813..875177b 100644
--- a/src/http/ngx_http_header_filter_module.c
+++ b/src/http/ngx_http_header_filter_module.c
@@ -270,6 +270,12 @@
             len += NGX_INT_T_LEN;
             status_line = NULL;
         }
+
+        if (status_line && status_line->len == 0) {
+            status = r->headers_out.status;
+            len += NGX_INT_T_LEN;
+            status_line = NULL;
+        }
     }
 
     clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);