Issue/fix broken superproperties

There was a bug with the way we were registering super properties that basically resulted in none of them being stored. I had incorrectly assumed that calling `registerSuperProperties` on the Mixpanel library with a dictionary would result in that dictionary being appended to the already existing super properties. Turns out it was actually wiping out the previous dictionary. On the iOS app we were able to get around this by reaching into the Mixpanel class and grab the currently stored super properties and use those as a baseline before registering a new one. Unfortunately this wasn't previously available but Mixpanel kindly patched it for us this morning allowing us to easily fix this bug.

cc @maxme