Tests: fixed proxy_cache_error.t with "tcp_nopush on".

Added "postpone_output 0;" to ensure that the backend will return
a response header immediately even if "tcp_nopush on;" is used in
the configuration (e.g., via TEST_NGINX_GLOBALS_HTTP).

Author: mdounin

proxy_cache_error.t

@@ -55,6 +55,7 @@ http {
         server_name  localhost;
 
         location / {
+            postpone_output 0;
             limit_rate 512;
             expires 1m;
         }