Fixed "return" with discarding invalid chunked body.

When ngx_http_discard_request_body() call was added to ngx_http_send_response(),
there were no return codes other than NGX_OK and NGX_HTTP_INTERNAL_SERVER_ERROR.
Now it can also return NGX_HTTP_BAD_REQUEST, but ngx_http_send_response() still
incorrectly transforms it to NGX_HTTP_INTERNAL_SERVER_ERROR.

The fix is to propagate ngx_http_discard_request_body() errors.

src/http/ngx_http_core_module.c

@@ -1660,8 +1660,10 @@ ngx_http_send_response(ngx_http_request_t *r, ngx_uint_t status,
     ngx_buf_t    *b;
     ngx_chain_t   out;
 
-    if (ngx_http_discard_request_body(r) != NGX_OK) {
-        return NGX_HTTP_INTERNAL_SERVER_ERROR;
+    rc = ngx_http_discard_request_body(r);
+
+    if (rc != NGX_OK) {
+        return rc;
     }
 
     r->headers_out.status = status;