Auto-convert unix 'time' field to SQL compatible Timestamp field.

[arohter]

We're importing mixpanel events into AWS Redshift, which unfortunately does not natively support unix timestamp format. This PR injects a new SQL Timestamp/DateTime format compatible field during export, which can be used directly when importing into a SQL database.

[erik]

Awesome, thank you! Definitely a good improvement to make.

Tests are breaking because this assumes that the "time" property is always specified. I think that this is a valid assumption to make, but I'm not totally sure after reading the docs... Any insight?

Could be safer to just check for nil before casting.

[erik]

Ah, found it in the HTTP spec

time: ... If this property is not included in your request, Mixpanel will use the time the event arrives at the server.

So that should be a fine assumption then. Could you add a new test that verifies the time outputs are correct?

[arohter]

I'm new to go, but I believe I added a decent nil and type check :)

[erik]

Great! Thanks again!