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

Created by: 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.

PR checklist

• Read the contribution guidelines.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
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