During closing a livechat session an error occur :
_get_visitor_leave_message() got an unexpected keyword argument 'speaking_with'
Because argument 'speaking_with' is pass in this method https://github.com/odoo/odoo/blob/14.0/addons/im_livechat/models/mail_channel.py#L182

, it should be operator.

closes odoo/odoo#70630

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>