Fixed serialization of Dictionary reference values
Revert "Fixed serialization of Dictionary reference values"

This reverts commit a4e4673df86f866ebd374ce7efe31feddf6a642e.
Merge branch 'master' of https://github.com/sharpdx/SharpDX
Fixed dictionary serialization of reference values

Dictionary serialization methods assumed that the IDataSerialization
type (either key or value) was always a value type. However
default(TKey/TValue) returns null for reference types.

I also added a Test case to show that this is now working as intended