[hotfix] encode the email message to utf8 before sending mail (#3785)

* [hotfix] encode the email message to utf8 before sending mail

* [minor] review fixes

frappe/email/queue.py

@@ -474,6 +474,7 @@ def prepare_message(email, recipient, recipients_list):
 
 		message = message.replace("<!--recipient-->", recipient)
 
+	message = (message and message.encode('utf8')) or ''
 	if not email.attachments:
 		return message