add httpAgent as right now the connection is not kept alive

[vrunoa]

Currently, the connection is not keep-alive over a test life, during some investigation at Sauce I notice this Appium(webdriver) client is closing TCP connections before any new command.
Here's some Wireshark TCP dump showing it:

👆🏻Before any new command(request) there's a [FIN, ACK]

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With these changes looking at TCP dumps we can see the socket is kept open, so there's no more resync before new requests 👇🏻

[vrunoa]

@imurchie here we go again 😃

[imurchie]

Awesome!

[imurchie]

I don't have the ability to publish this, but I've pinged @jlipps on Slack about it.

[vrunoa]

@imurchie @jlipps if I create a patch for an old version of wd, once we use internally to sauce, could be considered for publishing?

[imurchie]

@vrunoa Any idea about how to mitigate the problem this is causing in the Appium build?

For instance, https://travis-ci.org/appium/appium/jobs/634369625