[BUG] Experimental message endpoint is broken

[lucagobbi]

Describe the bug

The experimental message endpoint /message is currently broken, it throws an HTTP 500 error, due to missing user_id in stray_cat call. Thanks to @michealsaccone to have point that out. Issue just for tracking, PR coming asap.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'http://{your_cat_installation}/docs'
2. Click on '/message'
3. Click on execute
4. See error

Expected behavior
This endpoint should return a CatMessage object like websocket endpoints