Refactor makeRequest method

[michaeljdennis]

Fixes #63

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:

This PR reduces the size of the makeRequest method by moving some functionality out to other methods. This fixes the following code climate warning: https://codeclimate.com/github/sendgrid/php-http-client/lib/Client.php#issue_59f3f044c7fa44000100001a

[thinkingserious]

@michaeljdennis

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.

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[codecov]

Codecov Report

Merging #79 into master will increase coverage by 4.37%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   82.68%   87.05%   +4.37%     
==========================================
  Files           3        3              
  Lines         179      170       -9     
==========================================
  Hits          148      148              
+ Misses         31       22       -9

Impacted Files	Coverage Δ
lib/Client.php	84.67% <0%> (+5.21%)	⬆️

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[thinkingserious]

Hello @michaeljdennis,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX