Added support for custom http agent

[AsyncLegs]

Thus we will able to use custom HTTP agents such as https://github.com/koichik/node-tunnel or https://www.npmjs.com/package/https-proxy-agent

[coveralls]

Coverage decreased (-0.07%) to 84.804% when pulling 60d1b93 on AsyncLegs:custom-http-agent-support into 7918661 on ethereum:1.x.

[AsyncLegs]

Hi guys, is there any chances to merge these enhancements to version 1.x?

[nivida]

@cgewecke I've applied the requested changes as described above from me. I've noticed while improving the options handling that the existing HTTP provider test cases aren't that good as they could be but this has to get addressed with a separate PR.

[cgewecke]

@nivida Is it a problem to add a simple test using the new option though?

To make sure the changes work as expected and the option is described by the tests in case someone wants to see a working example.

Also WDYT about adding withCredentials and agent to the usage example in the web3-providers-http README?

[nivida]

@nivida Is it a problem to add a simple test using the new option though?
To make sure the changes work as expected and the option is described by the tests in case someone wants to see a working example.

I will add those missing test cases 👌

Also WDYT about adding withCredentials and agent to the usage example in the web3-providers-http README?

Yep, I will add those options to the README.

Btw.: An issue is existing to add and improve the provider documentations to the web3.js documentation on readthedocs (#3117).

[cgewecke]

@nivida Looks good! Nice.

There's a .only on the new test which is stopping the complete test suite from running. (Tried to commit it but I don't have permission on this branch).