Fix csharp-netcore enum var name underscore and space handling

[sbergen]

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Fixes #8212

It seems I do not have the required permissions to perform the last step in the checklist.

[wing328]

cc @mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02)

[sbergen]

@wing328 Thanks for adding those mentions! I found the instructions a bit unclear here, as I'm used to using the Reviewers feature for requesting review, and assumed that was the expected method also here. Perhaps something like this could be clearer?

If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

| If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

Looks good to me. I wonder if you can submit a PR to revise the wording when you've time.

[wing328]

@sbergen thanks for the fix and the adding the tests.