Can't pass an object to cf.split

[rclark]

Because of the toString() here, if you pass a object (e.g. a "Ref") to cf.split(), it gets turned into [object Object].

> cf.split(',', cf.ref('SomeParameter'))
{ 'Fn::Split': [ ',', '[object Object]' ] }

cc @xrwang

[xrwang]

eeps ! over on https://github.com/mapbox/cloudfriend/pull/18