Track can't handle keys as values.

[JustinC474]

Currently if you pass in keys as values.. ie:

(segment-lib/track client user-id event {:old-plan :starter :new-plan :enterprise}

the keys old-plan and new-plan are discarded and the following warn messages are printed:

WARN com.github.segmentio.models.Props Key old-plan value :starter not allowed because it is not of type String, Integer, Double, Boolean, or Date.
WARN com.github.segmentio.models.Props Key new-plan value :enterprise not allowed because it is not of type String, Integer, Double, Boolean, or Date.

These should just be converted to strings.