Added the ability for users to specify a CA bundle for HTTPS requests

[barrybarrette]

Proposal to Issue #481

Right now there is no way for a user to specify a CA bundle to replace the default one that request ships with.

According to the request docs you can specify it in the options object passed to request.

This change offers a simple solution by setting a path to your CA bundle as an environment variable:
TWILIO_CA_BUNDLE=/path/to/cert.pem

Test cases have been included to ensure that the specified CA bundle is used, and that the original behavior of not including it in options remains unchanged if it is not specified.

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

[childish-sambino]

One request to update the docs. But overall LGTM. Nice work! (assuming you tested this locally and it resolves the issue you're having).

[barrybarrette]

One request to update the docs

Can you point me to a specific place in the docs you'd like me to update? I'm not too familiar with this module yet.

assuming you tested this locally and it resolves the issue you're having

Yes it works like a charm locally 😄

[childish-sambino]

@whitebarry I think sticking a block in/under here is fine: https://github.com/twilio/twilio-node#sample-usage

[barrybarrette]

Ok added some info to the example file and also the README block for environment variables. I'm not exactly clear on the usage of the first 2 you mentioned so I left the descriptions for those as TODOs, hope that's OK.

[childish-sambino]

@whitebarry Updated the README.

One thing that did cross my mind: since the CA payload is added to every API request, it would be better to cache the file contents on the first load so subsequent requests don't need to read from disk. Not a blocker for this PR; just an optimization.

[barrybarrette]

since the CA payload is added to every API request, it would be better to cache the file contents on the first load

I couldn't agree more. My latest change addresses this with a supporting test case.

[childish-sambino]

👍

[childish-sambino]

FYI, next planned release is on 2019-10-02.