[c-sharp] Fix default values with discriminator

[tomelfring]

Fixes a part of #6225, where the default values aren't registered correctly due to a mismatch in comparing between name and baseName in the class CSharpClientCodegen.java.

Chosen for comparing name because that's the value used more in the same file.

Testcase is already in the samplefiles at:
samples/client/petstore/csharp/OpenAPIClient/src/Org.OpenAPITools/Model/Cat.cs
The default value of className is now set to "Cat" instead of default(string).

Additional testcase is provided in #6225.

To fix #6225

PR checklist

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@mandrean (2017/08), @jimschubert (2017/09) ❤️ @frankyjuang (2019/09) @shibayan (2020/02)

[wing328]

Closed via #7296