Upstream: NGX_HTTP_GATEWAY_TIME_OUT after upstream timeouts.

There is no real difference from previously used 0 as NGX_HTTP_* will
become 0 in ngx_http_upstream_finalize_request(), but the change
preserves information about a timeout a bit longer.  Previous use of
ETIMEDOUT in one place was just wrong.

Note well that with cacheable responses there will be a difference
(code in ngx_http_upstream_finalize_request() will store the error
in cache), though this change doesn't touch cacheable case.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 0174d4b..1ecc9be 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2018,7 +2018,7 @@
 
     if (rev->timedout) {
         ngx_connection_error(c, NGX_ETIMEDOUT, "upstream timed out");
-        ngx_http_upstream_finalize_request(r, u, NGX_ETIMEDOUT);
+        ngx_http_upstream_finalize_request(r, u, NGX_HTTP_GATEWAY_TIME_OUT);
         return;
     }
 
@@ -2514,7 +2514,7 @@
 
     if (upstream->read->timedout || upstream->write->timedout) {
         ngx_connection_error(c, NGX_ETIMEDOUT, "upstream timed out");
-        ngx_http_upstream_finalize_request(r, u, 0);
+        ngx_http_upstream_finalize_request(r, u, NGX_HTTP_GATEWAY_TIME_OUT);
         return;
     }
 
@@ -2701,7 +2701,7 @@
 
     if (c->read->timedout) {
         ngx_connection_error(c, NGX_ETIMEDOUT, "upstream timed out");
-        ngx_http_upstream_finalize_request(r, u, 0);
+        ngx_http_upstream_finalize_request(r, u, NGX_HTTP_GATEWAY_TIME_OUT);
         return;
     }