[MINOR] redirect: in prefix mode a "/" means not to change the URI

If the prefix is set to "/", it means the user does not want to alter
the original URI, so we don't want to insert a new slash before the
original URI.

(cherry-picked from commit 02a35c74942c1bce762e996698add1270e6a5030)
diff --git a/tests/test-redirect.cfg b/tests/test-redirect.cfg
index 780132b..582a069 100644
--- a/tests/test-redirect.cfg
+++ b/tests/test-redirect.cfg
@@ -26,8 +26,8 @@
 	redirect   prefix   /pfx/test code 302 if url_test2
 	redirect   prefix   /pfx/test code 303 drop-query if url_test3
 
-	redirect   location /test4 code 302 set-cookie   SEEN=1 if url_test4 !seen
-	redirect   location /      code 302 clear-cookie SEEN=  if url_test4 seen
+	redirect   prefix   /  code 302 set-cookie   SEEN=1 if url_test4 !seen
+	redirect   location /  code 302 clear-cookie SEEN=  if url_test4 seen
 
 	### unconditional redirection
 	#redirect   location https://example.com/ if TRUE