treehouses message slack sendto (fixes #1810)

[warthog16]

fixes #1810

[sarahrams]

root@raspberrypi:~/cli# ./cli.sh message slack sendto messaging test
token:
token: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
channel: messaging
message: test
{"ok":true,"channel":"C017XS65KRB","ts":"1598912011.000400","message":{"bot_id":"B01AJ9TCRG8","type":"message","text":"test","user":"U019H48FQ4D","ts":"1598912011.000400","team":"T017XS656RK","bot_profile":{"id":"B01AJ9TCRG8","deleted":false,"name":"mytest","updated":1598911849,"app_id":"A019UEALHJN","icons":{"image_36":"https:\/\/a.slack-edge.com\/80588\/img\/plugins\/app\/bot_36.png","image_48":"https:\/\/a.slack-edge.com\/80588\/img\/plugins\/app\/bot_48.png","image_72":"https:\/\/a.slack-edge.com\/80588\/img\/plugins\/app\/service_72.png"},"team_id":"T017XS656RK"}},"warning":"missing_charset","response_metadata":{"warnings":["missing_charset"]}}root@raspberrypi:~/cli#

It's working fine