Update MixpanelType.swift
Update MixpanelType.swift

Hello,

`Faced with the issue when String comes from ObjC as a NSString.

Behaviour:
(lldb) po ("Test" as NSString) as? MixpanelType
nil

(lldb) po ("Test" as String) as MixpanelType
"Test"`

Expected:
`(lldb) po ("Test" as NSString) as MixpanelType
Test

(lldb) po ("Test" as String) as MixpanelType
"Test"`
mixpanel/mixpanel-swift,mixpanel/mixpanel-swift,mixpanel/mixpanel-swift,mixpanel/mixpanel-swift,mixpanel/mixpanel-swift