Avoid clearing a header list if it is being appended to.
Avoid clearing a header list if it is being appended to.

I think this resolves a copy/paste error that was introduced in
4a80ae527df9aa36fe50fac0878207a31d4d6b72.

The logic for `message_add_addresses` is that it is supposed to just
be appending to a list of addresses, rather than trying to re-merge a
series of addresses.

It looks like the function's framing was copy/pasted from
`message_update_addresses`, which does the more complete
re-build/re-merge.

However, `message_update_addresses` starts with an initial
`internet_address_list_clear`, and then proceeds to cycle through the
entire list of headers, trying to collect the ones that match the
field name, so clearing the address list initially makes sense.

For `message_add_addresses` though, the goal is just to append to the
current list that already exists, so the list should not be initially
cleared.

Fixes: #129

jstedfast/gmime,jstedfast/gmime,jstedfast/gmime