dotnet-svcutil not reusing netstandard types in wcf client code generation

[imcarolwang]

dotnet-svcutil will generate types such as TargetNameSpace.HttpStatusCode if service contract referenced to System.Net.HttpStatusCode, type HttpStatusCode is now part of netstandard2.0 and dotnet-svcutil-lib.dll is already targeting to netstandard2.0, we should reuse existing types of .NetCore platform instead of generating new one.