Inconsistent/wrong implementations of Keys property by guarded dictionaries

[eskesparsoe]

GuardedDictionary implements the Keys property as 'return dict.Keys;' with a TODO:
//TODO: guard with a read-only wrapper? Probably so!

GuardedSortedDictionary implements the Keys property as 'return null;'

The implementations should be fixed, in particular the one in GuardedSortedDictionary.

The implementations should probably wrap the keys of the wrapped dictionary in a GuardedCollection / GuardedSorted.

[ondfisk]

Fixed in 3.0.