Add a helper returns the response body as a hash

[diegoc-am]

I think the body is already a json string, therefore I only created a method to allow it to be hash

[SendGridDX]

All committers have signed the CLA.

[diegoc-am]

Built the gem and ran the tests against sendgrid-ruby, nothing failed

sendgrid-ruby git:master ❯ bundle exec rake test                                                                                                             ⏎
/home/diego/.rbenv/versions/2.4.1/bin/ruby -I"lib:test" -I"/home/diego/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-0.9.6/lib" "/home/diego/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rake-0.9.6/lib/rake/rake_test_loader.rb" "test/sendgrid/test_sendgrid-ruby.rb" "test/sendgrid/helpers/mail/test_category.rb" "test/sendgrid/helpers/mail/test_mail.rb" "test/sendgrid/helpers/mail/test_personalizations.rb" 
Activating Prism (~20 seconds)
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Run options: --seed 8574

# Running:

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Finished in 14.516840s, 17.6347 runs/s, 18.3235 assertions/s.

256 runs, 266 assertions, 0 failures, 0 errors, 0 skips
Prism shut down

[diegoc-am]

I think I've already signed the CLA before

[thinkingserious]

Hello @belfazt,

Thanks again for the PR!

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

Team SendGrid DX