Update C# client dependency

[wing328]

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Update C# client dependency (nunit, jsonsubtypes, Newtonsoft.Json, etc)

cc @mandrean (2017/08), @jimschubert (2017/09) 

[jimschubert]

I support upgrading!

Did you test this across supported framework versions? I recall there was an issue with newer versions of either JSON.net or the subtypes package (possibly both), which had issues with one or more of our framework options.

The errors were only presented at runtime.

[wing328]

Did you test this across supported framework versions?

I didn't. I only leverage the AppVeyor to test the change at the moment. Agreed with you that we need to test more thoroughly with different frameworks.

[wing328]

Tests running fine locally but in Appveyor, it couldn't find nunit dll:

Project "C:\projects\openapi-generator\samples\client\petstore\csharp\OpenAPIClient\Org.OpenAPITools.sln" (1) is building "C:\projects\openapi-generator\samples\client\petstore\csharp\OpenAPIClient\src\Org.OpenAPITools.Test\Org.OpenAPITools.Test.csproj" (3) on node 1 (default targets).
PrepareForBuild:
  Creating directory "bin\Debug\".
  Creating directory "obj\Debug\".
ResolveAssemblyReferences:
  Primary reference "nunit.framework".
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(2110,5): warning MSB3245: Could not resolve this reference. Could not locate the assembly "nunit.framework". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors. [C:\projects\openapi-generator\samples\client\petstore\csharp\OpenAPIClient\src\Org.OpenAPITools.Test\Org.OpenAPITools.Test.csproj]
          For SearchPath "{HintPathFromItem}".
          Considered "..\..\packages\NUnit.3.11.0\lib\nunit.framework.dll", but it didn't exist.
          For SearchPath "{TargetFrameworkDirectory}".
          Considered "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\nunit.framework.winmd", but it didn't exist.
          Considered "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\nunit.framework.dll", but it didn't exist.
          Considered "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\nunit.framework.exe", but it didn't exist.

Anyone has seen something similar before?

[wing328]

NUnit 3.11 is the latest version: https://nunit.org/download/

[wing328]

@jimschubert finally I'm able to fix the issue and all tests passed.