allowing handling 400 errors via proxy_pass

src/http/ngx_http_request.c

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index 73eeeb0..5ce2e2a 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -979,7 +979,7 @@
         c->error = rev->error;
         c->log->action = "sending response to client";
 
-        ngx_http_finalize_request(r, NGX_HTTP_BAD_REQUEST);
+        ngx_http_finalize_request(r, NGX_HTTP_CLIENT_CLOSED_REQUEST);
         return NGX_ERROR;
     }