chore: [issue#52] Added unit tests to check some files exist

[luan-cestari]

Description of the change:
This PR fulfill the request from issue #52 , which asked to create tests to validate if some files exist

Reason for the change:
The reason behind the request is guarantee that some basic files like README.MD will always be present int he project, so, if it is erased by mistake, the tests will not pass and it will be explicit the problem, without any human manual validation

Link to original source:
The actual source https://github.com/sendgrid/nodejs-http-client/blob/90ca4ebb6e0e6b59bd554908105f66114ad5a8f6/test/test.js

Checklist

Make sure all of these items are complete, or else the PR will be ineligible for a code review.

• Code passes all existing tests the new tests do not pass as it is expected mentioned in Add unittest to check for specific repo files #52
• Any new functionality added includes new unit tests in test/test.js
• Create or update example code to show the new functionality in action.
• All code, branch, and git naming and style conventions are followed (see CONTRIBUTING.md)
• Feature branch has been rebased off of the latest master branch. ( see CONTRIBUTING.md ).

[SendGridDX]

All committers have signed the CLA.

[mbernier]

BEAUTIFULLY done!

This is awesome. Thank you so much!!

Just so you know, we have this same issue on another Node library if you're interested:

• Add unittest to check for specific repo files sendgrid-nodejs#565

[thinkingserious]

Hello @luan-cestari,

Thanks again for the PR!

We want to show our appreciation by sending you some swag. Could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[luan-cestari]

Thank you guys =) I hope I can help with other issues in the future