Fix "method_count" issue in sendgrid/helpers/mail/personalization.py

[thinkingserious]

Personalization has 24 functions (exceeds 20 allowed). Consider refactoring.

https://codeclimate.com/github/sendgrid/sendgrid-python/sendgrid/helpers/mail/personalization.py#issue_59f5822373ac790001000072

[defaults]

@thinkingserious Is not this a duplicate of #487
fixing some of the codeclimate issue for personalization.py in #493