Attempting to set substitutions on Personalization helper causes infinite regress #582

Issue Summary

Attempting to set substitutions on a Personalization helper directly (rather than with add_substitution) throws a RecursionError.

Steps to Reproduce

from sendgrid.helpers.mail import Personalization
p = Personalization()
p.substitutions = [{'a': 0}, {'b': 1}]

This throws a RecursionError. The offending line is personalization.py#L119.

I will open a PR as this is an easy fix.

Technical details:

• sendgrid-python Version: 5.3.0
• Python Version: 3.6