[mixpanel] Pull Request #34: Map CapitalCase special properties to mixpanel

[SegmentDestinationsBot]

Migrated from #34 by @ErikAGriffin

Previously only camel case special properties would map to their Mixpanel equivalents,
eg. "name" maps to "$name" and "email" to "$email". This causes issues when a
CapitalCase trait naming convention is used. This change allows for all special properties
to be properly mapped to their Mixpanel equivalents, eg. "Name" to "$name" and "Email"
to "$email".