[csharp-netcore] Fix: Add only first Authorization header

[Anakael]

Apply fix from #11272 to httpclient library

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[wing328]

cc @mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

[wing328]

LGTM. Thanks for the fix.