sendTextMessage: take into account payload type

Currently, the daemon lie a bit about sendTextMessage. Indeed, it
takes a map (string, string) like {{datatype: msg}} but the datatype
is ignored.

It can be problematic if a client wants to manipulate a custom datatype
because the daemon will replace the datatype by "plain/text". Now,
an ImMessage can have a datatype which permits to not lie in the api
and manipulate the datatype the user wants.

Change-Id: I1ec5a34082337570bf43231078fc894304b00182
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>