Don't forget the "to" header when restrict parsing to certain headers

We recently started discarding files as "not email" if they have none
of Subject, From, nor To. Apaprently, my mail collection contains a
number of messages that I sent, that are saved without Subject and
From, (perhaps these were drafts?).

Anyway, it's fortunate I had those since they alerted me to this bug,
where we were not parsing the "To" header in some cases.

database.cc

@@ -514,6 +514,7 @@ notmuch_database_add_message (notmuch_database_t *notmuch,
 					   "message-id",
 					   "references",
 					   "subject",
+					   "to",
 					   (char *) NULL);
 
     try {