Bug 4912: same-name notes being appended instead of replaced (#393)

Bug 4912: same-name notes being appended instead of replaced (#393)

When auth helper replies with clt_conn_tag=foo (or any KV pair) and
then later in future replaces it with clt_conn_tag=bar. The new pair
was getting appended instead of getting replaced.

i.e. notes would contain two KV pairs:
clt_conn_tag=foo
clt_conn_tag=bar

This fixes it and also fixes:
https://bugs.squid-cache.org/show_bug.cgi?id=4912