Threw the correct exception when the generic definition type 'Dictionary<,>' was requeted.
Supproted creating a specimen of the 'Collection<T>' type.
Renamed the method 'TryCast' to 'TryCastTo'.
Renamed FixedGeneratorSpecimenProvider to SpecimenCollectionSpecimenProvider.
Refactored some classes.
Supproted creating a specimen of the 'ReadOnlyCollection<T>',  'ObservableCollection<T>' and 'ReadOnlyObservableCollection<T>' types.