Tests: adjusted proxy_cache_error.t read timeout.

A previous limit could not be enough to get response header on slow hosts.
It is encreased to still catch alerts on old versions before 93abb5a855d6.

proxy_cache_error.t

diff --git a/proxy_cache_error.t b/proxy_cache_error.t
index 2b12db7..2ac2d7e 100644
--- a/proxy_cache_error.t
+++ b/proxy_cache_error.t
@@ -46,7 +46,7 @@
             proxy_pass    http://127.0.0.1:8081;
             proxy_cache   NAME;
 
-            proxy_read_timeout 100ms;
+            proxy_read_timeout 500ms;
         }
     }
 
@@ -70,7 +70,7 @@
 
 ###############################################################################
 
-# make a HEAD request; as cache is enabled, nginx convert HEAD to GET
+# make a HEAD request; since cache is enabled, nginx converts HEAD to GET
 # and will set u->pipe->downstream_error to suppress sending the response
 # body to the client