Issue Summary

.add_header() cannot be used because python raises a TypeError exception.

Steps to Reproduce

1. Create an instance of Mail mail = Mail( Email(from_email), subject, Email(to_emails), Content('text/html', message) )
2. Add a header to the instance
   mail.add_header( {'Reply-To' : 'user@server.ext, user@server.ext'} )
3. Send the Request
   response = sg.client.mail.send.post(request_body=mail.get())

Stack Trace

File "mail.py", line 63, in get

headers.update(key**.get()**)

TypeError: get expected at least 1 arguments, got 0