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.

lisp/gnus/message.el

@@ -4593,10 +4593,12 @@ This function could be useful in `message-setup-hook'."
 	    (message-insert-courtesy-copy
 	     (with-current-buffer mailbuf
 	       message-courtesy-message)))
-          ;; Let's make sure we encoded all the body.
-          (cl-assert (save-excursion
-                       (goto-char (point-min))
-                       (not (re-search-forward "[^\000-\377]" nil t))))
+          ;; If this was set, let `sendmail-program' handle the Unicode
+          (unless message-inhibit-body-encoding
+            ;; Let's make sure we encoded all the body.
+            (cl-assert (save-excursion
+                         (goto-char (point-min))
+                         (not (re-search-forward "[^\000-\377]" nil t)))))
           (mm-disable-multibyte)
 	  (if (or (not message-send-mail-partially-limit)
 		  (< (buffer-size) message-send-mail-partially-limit)