test: test enhancements

[peter279k]

Resolves #119.

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:

• Test enhancement.

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #120 into master will decrease coverage by 3.75%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #120      +/-   ##
============================================
- Coverage     88.00%   84.24%   -3.76%     
- Complexity        0       56      +56     
============================================
  Files             4        2       -2     
  Lines           175      165      -10     
============================================
- Hits            154      139      -15     
- Misses           21       26       +5     

Impacted Files	Coverage Δ	Complexity Δ
lib/Client.php	81.56% <0.00%> (-3.76%)	47.00% <0.00%> (+47.00%)	⬇️
test/unit/MockClient.php
lib/Exception/InvalidRequest.php

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[childish-sambino]

👍