[csharp-netcore] fix: url escaping

[Anakael]

HttpUtility.UrlEncode converts space to + instead of %20 which is leads to error in parsing on server side.
Uri.EscapeDataString is right methods for escaping.

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[Anakael]

@mandrean @frankyjuang @shibayan @Blackclaws @lucamazzanti

[wing328]

LGTM. Thanks for the fix.