Handle the special property IP correctly when setting it on people profiles

Mixpanel expect an optional IP property differently in their People REST API [1] and we have made a small change to reflect this. If an `ip` property is passed to `people.set`, it will be correctly extracted and passed as `$ip` to Mixpanel.

We are using this fork in production and have created thousands of people profiles since it has been rolled out and the profiles correctly get `Country`, `Region` and `City` properties set on them based on the IP address.

[1] https://mixpanel.com/docs/people-analytics/people-http-specification-insert-data