Attempt to fix issues in sendgrid/helpers/mail/mail.py

[galihmelon]

Fixes

Fixes #457
Fixes #458

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Another attempt to decrease Cognitive Complexity of function
• Another attempt to deduplicate similar blocks of code
• Revert private variables to be public if they do not need mutations during setting/getting

[SendGridDX]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #496 into master will decrease coverage by 1.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
- Coverage      83%   81.83%   -1.18%     
==========================================
  Files          30       30              
  Lines        1024      936      -88     
  Branches      160      133      -27     
==========================================
- Hits          850      766      -84     
  Misses         84       84              
+ Partials       90       86       -4

Impacted Files	Coverage Δ
sendgrid/helpers/mail/mail.py	100% <100%> (+2.48%)	⬆️

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[thinkingserious]

@galihmelon

We have not been able to merge your Pull Request, but because you are awesome - we wanted to make sure you could still get a SendGrid Hacktoberfest shirt.

Please go fill out our swag form before Nov 5th and we will send the shirt! (We know that you might have tried this before and it didn’t work, sorry about that!)

You have till Nov 5th to fill out this form in order to get the Hacktoberfest shirt!

Thank you for contributing during Hacktoberfest! We hope to see you in the repos soon! Just so you know, we always give away a SendGrid shirt for your first ever non-Hacktoberfest PR that gets merged.

[thinkingserious]

Thanks again for helping improve this SDK!

We have started to execute on PR #347 (Python Mail Helper Refactor) and have integrated your work into this branch. Please follow along at PR #347 to follow our progress.

With Best Regards,

Elmer