lisp/gnus/message.el (message-send-mail): Don't wrongly assert on Unicode msg

Using this setting:

    (setq message-send-mail-function 'message-send-mail-with-sendmail)
    (setq sendmail-program "msmtp")

the message seding is handled by an external program, so no Unicode
encoding is performed in Emacs. Thus Emacs must not assert that the
Unicode encoding was performed.
1 job for master in 3 seconds
Status Job ID Name Coverage
failed #1991


Name Stage Failure
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)