Upstream: consistent error handling after u->input_filter_init().

In all cases ngx_http_upstream_finalize_request() with NGX_ERROR now used.
Previously used NGX_HTTP_INTERNAL_SERVER_ERROR in the subrequest in memory
case don't cause any harm, but inconsistent with other uses.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index a6397c9..50c41e3 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1682,8 +1682,7 @@
     }
 
     if (u->input_filter_init(u->input_filter_ctx) == NGX_ERROR) {
-        ngx_http_upstream_finalize_request(r, u,
-                                           NGX_HTTP_INTERNAL_SERVER_ERROR);
+        ngx_http_upstream_finalize_request(r, u, NGX_ERROR);
         return;
     }