Allow arrays (lists) properties

I tried to send a property as a list data-type in Mixpanel. See:

https://mixpanel.com/docs/properties-or-segments/property-data-types

But I was getting an error:

AttributeError: 'list' object has no attribute 'decode'

This pull request fixes libsaas to be able to bypass a python list type object when encoding property values before converting the payload to JSON.

Cheers!