Scripting: mavlink:send_chan does not send anything

[twistedwings]

Bug report

mavlink:send_chan does not send any data.

also reported here

Issue details
can be reproduced with this example:
libraries/AP_Scripting/examples/MAVLink_Commands.lua
No ACK is sent.

Version
4.5.6
4.6.0

Platform
[ ] All
[ ] AntennaTracker
[X ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Airframe type
X

Hardware type
SITL

Logs
Please provide a link to any relevant logs that show the issue

[rmackay9]

I have not tried sending command-long messages but other messages can be sent and I've recently done it with the copter-slung-payload.lua script included in this PR #27783

[twistedwings]

it works.
Made a new test with a minimal lua.