This kind of map is used for MIME headers (e.g. for HTTP, see
vibe.inet.message.InetHeaderMap), or for form data
(vibe.inet.webform.FormFields). Note that the map can contain fields with
the same key multiple times if addField is used for insertion. Insertion
order is preserved.
Note that despite case not being relevant for matching keyse, iterating
over the map will yield the original case of the key that was put in.