MINOR: cache: Replace the "process-vary" option's expected values

Replace the <0/1> expected values of the process-vary option by a more
usual <on/off> pair.
diff --git a/reg-tests/cache/vary_accept_encoding.vtc b/reg-tests/cache/vary_accept_encoding.vtc
index 9b55dc9..afd2cfb 100644
--- a/reg-tests/cache/vary_accept_encoding.vtc
+++ b/reg-tests/cache/vary_accept_encoding.vtc
@@ -98,7 +98,7 @@
                total-max-size 3
                max-age 20
                max-object-size 3072
-               process-vary 1
+               process-vary on
 } -start