Substitutions allow non-strings for values

[tylerreinhart]

Issue Summary

API only allows strings for substitution values. This library will allow substitutions of non-strings and will fail on send attempt.

Steps to Reproduce

1. message = Mail()
2. personalization = Personalization()
3. personalization.add_to(Email('test@email.com'))
4. personalization.add_substitution(Substitution('-test-, 1))
5. message.add_personalization(personalization)
6. sg.client.mail.send.post(request_body=message.get()) 400 returned because of int in substitution json

Should an error be thrown when attempting to create a non-string substitution?

[thinkingserious]

Hi @tylerreinhart,

Thanks for reporting this issue! I've added it to the backlog for execution.

I think we should make the conversion in the library and log a warning.

With Best Regards,

Elmer