REGTEST: fix the race conditions in digest.vtc

A "Connection: close" header is added to responses to avoid any connection
reuse. This should avoid any "HTTP header incomplete" errors.
diff --git a/reg-tests/converter/digest.vtc b/reg-tests/converter/digest.vtc
index fab173d..e911ff4 100644
--- a/reg-tests/converter/digest.vtc
+++ b/reg-tests/converter/digest.vtc
@@ -7,7 +7,7 @@
 
 server s1 {
 	rxreq
-	txresp
+	txresp -hdr "Connection: close"
 } -repeat 2 -start
 
 haproxy h1 -conf {