Fixing up HandleNull in JsonSerializer.Read to reset the property for dictionaries. (dotnet/corefx#38704)
* Fixing up HandleNull in JsonSerializer.Read to reset the property for dictionaries.
* Adding regression tests for dotnet/corefx#38565
* Fixing HandleDictionary in JsonSerializer.Write to respect IgnoreNullValues
* Updating the Regression38565_Serialize test to not expect null
* Responding to PR feedback and adding a regression test for dotnet/corefx#38557
Commit migrated from https://github.com/dotnet/corefx/commit/
78e00d4b0ad5b390c9a54431c632062392c3ed7e