Add tests for Sendgrid's http client

[c-w]

Resolves #120

[codecov-io]

Codecov Report

Merging #121 into master will increase coverage by 0.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   91.29%   91.67%   +0.38%     
==========================================
  Files          22       22              
  Lines         930      925       -5     
==========================================
- Hits          849      848       -1     
+ Misses         81       77       -4

Impacted Files	Coverage Δ
opwen_email_server/services/sendgrid.py	82.02% <ø> (+4.36%)	⬆️
opwen_email_server/utils/log.py	88.23% <0%> (-1.18%)	⬇️

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