Upstream: setting u->header_sent before ngx_http_upstream_upgrade().

Without u->header_sent set a special response might be generated following
an upgraded connection.  The problem appeared in 1ccdda1f37f3 (1.5.3).
Catched by "header already sent" alerts in 1.5.4 after upstream timeouts.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 652222f..f4221ed 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2105,6 +2105,8 @@
         return;
     }
 
+    u->header_sent = 1;
+
     if (u->upgrade) {
         ngx_http_upstream_upgrade(r, u);
         return;
@@ -2131,8 +2133,6 @@
         }
     }
 
-    u->header_sent = 1;
-
     if (r->request_body && r->request_body->temp_file) {
         ngx_pool_run_cleanup_file(r->pool, r->request_body->temp_file->file.fd);
         r->request_body->temp_file->file.fd = NGX_INVALID_FILE;