Fix Personalization.substitutions setter

[richardnias]

Trying to set substitutions directly rather than with add_substitution was causing an infinite regress

Fixes #582

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:

• the setter for Personalization.substitutions was incorrectly referencing self.subtitutions rather than self._substitutions
• I added a very simple testcase.

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #583 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #583     +/-   ##
=========================================
+ Coverage   86.51%   86.61%   +0.1%     
=========================================
  Files          32       32             
  Lines         979      979             
  Branches      131      131             
=========================================
+ Hits          847      848      +1     
+ Misses         68       67      -1     
  Partials       64       64

Impacted Files	Coverage Δ
sendgrid/helpers/mail/personalization.py	92.68% <100%> (+1.21%)	⬆️

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[thinkingserious]

Hello @richardnias,

Thanks again for the PR!

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