* Fix many version checks to be based on Xcode version instead of iOS version.
* Added/fixed a few expected values according to platform version to match behavior in older macOS versions.
We cannot use generics in native signatures but, with some care (and
generator work), we can still expose nullable on our public delegate API.
AUImplementorStringFromValueCallback is presently the only case of such
an API.
reference:
https://bugzilla.xamarin.com/show_bug.cgi?id=42699
Rolf Bjarne Kvinge
Sebastien Pouliot
Timothy Risi
Alex Soto