Fix: double underscore in case uppercase character is preceeded by an underscore

[andreycha]

Problem
Currently property like User_Id is serialized as user__id (note double underscore).

Changes

• Fix to produce only one underscore
• Add tests for snake case

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• Start Time: 2022-08-22T06:58:08.470+0000

• Duration: 14 min 23 sec

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Test	Results
Failed	0
Passed	84
Skipped	0
Total	84

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[Mpdreamz]

LGTM thanks for catching this @andreycha ! Waiting to sort out the CLA before merging.

Can you retarget this PR to the main branch?

[andreycha]

Done.

Probably it's worth marking somewhere that once dotnet/runtime#782 is released (seems to be part of .NET 8) custom naming policy can be dropped.