Runtime System.Text.Json | Refactor and Add Test cases about JsonHelper

[KimKiHyuk]

Merged

• Tracking: #3
• Issue: https://github.com/devrel-kr/oss-contributhon-2020-dotnetcore-sdk/issues/17
• PR: https://github.com/dotnet/runtime/pull/40759

[KimKiHyuk]

@justinyoo 머지되었습니다!!

Labels : adadaDot, runtime assign : @KimKiHyuk

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