Add unicode escaping
Add unicode escaping

Following https://sendgrid.com/docs/API_Reference/SMTP_API/using_the_smtp_api.html#-Sending-Unicode-in-the-SMTP-API
Copied from https://github.com/SendGridJP/smtpapi-ruby/blob/master/lib/smtpapi.rb#L143-154

Ghrind/sendgrid