NSCache SetObjectForCache Typo

[drasticactions]

The name for NSCache.SetObjectForCache was typed to SetObjectforCache for the NSObject, NSObject implementation, which is not consistant with the other implementations in this class that have the correct casing.

https://github.com/xamarin/xamarin-macios/blob/b055fa4b10c4b770932d0aac13a45be5c02fbbe4/src/foundation.cs#L975-L976

[rolfbjarne]

Fix in progress.