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

[Anakael]

This PR fix problem in csharp-netcore client with invalid(multiple) authorization header if more than one security present in spec file.

Steps to reproduce:

1. Create new C# web project with bearer auth scheme which consumes access_token and put Authorize attribute on test controller
2. Install Swashbuckle to project
3. Add Bearer and OAuth schemes to swagger via AddSecurityRequirement
4. Generate csharp-netcore client against spec
5. Put AccessToken in Configuration for generated client
6. Call request on generated client <-- 401 here

Solution:

Pass only one Authorization header. Which one will be chosen from first met scheme.

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[Anakael]

@mandrean @frankyjuang @shibayan @Blackclaws @lucamazzanti

[wing328]

Appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/42153714