Fix equality comparer used in ReferencedObjectCollection's overflow dictionary

[ReubenBond]

ReferencedObjectCollection uses a hybrid approach to storing object references: an array is used for tracking the first 64 reference objects and a Dictionary<object, uint> is used for subsequent objects.

That dictionary uses ReferenceEqualsComparer.Default to check for equality. ReferenceEqualsComparer doesn't explicitly define a Default property, though, instead it inherits from EqualityComparer<object>. This was the mistake: the Default property was returning the default equality comparer which means we do not have reference equality. Eg, It can tell you that a Uri and string are equal, as it was in the test case which @mcm2020 provided.

Microsoft Reviewers: Open in CodeFlow