The Mail Helper class should use properties

[thinkingserious]

Issue Summary

https://github.com/sendgrid/sendgrid-python/blob/master/sendgrid/helpers/mail/mail.py

[denis90]

I'd like to work on this issue

[thinkingserious]

Hi @denis90,

Thank you!

Could you please take a moment to sign our CLA so that we can merge your changes?