test: add Simplecov Local Enhancements

[RolandBurrows]

Short description of what this PR does:

As a contributor, I'd like to know what baseline code coverage is and how my enhancements affect it.

It was actually fairly difficult to find a good location to insert the SimpleCov.start and keep the results consistent. But after several hours, the final state of this PR seems to be the best.

No documentation was updated because code coverage is now part of the default rake run, with no further action required.

sendgrid-ruby$ rake

<spec debug info>
Finished in 5.17 seconds (files took 0.51309 seconds to load)
54 examples, 0 failures
Coverage report generated for RSpec, Unit Tests to ../sendgrid-ruby/coverage. 1074 / 1111 LOC (96.67%) covered.

<test debug info> 
Finished in 24.280586s, 11.5318 runs/s, 12.0673 assertions/s.
280 runs, 293 assertions, 0 failures, 0 errors, 0 skips
Coverage report generated for Unit Tests to ../sendgrid-ruby/coverage. 843 / 936 LOC (90.06%) covered.

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.
• (N/A) I have added tests that prove my fix is effective or that my feature works
• (N/A) I have added necessary documentation about the functionality in the appropriate .md file
• (N/A) I have added in line documentation to the code I modified