[BUG] [csharp-netcore] DefaultHeader in Configuration are ignored

Created by: mario-d-s

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)? N/A
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

The Configuration of the ApiClient has a DefaultHeader property (which, tangentially, shoud be named DefaultHeaders) - an IDictionary<string, string>.

However, adding anything to this dictionary appears to have no effect. Those "default headers" are not sent with any request.

openapi-generator version

4.1.0-SNAPSHOT via Docker.

OpenAPI declaration file content or url

N/A - it occurs with any generated SDK

Command line used for generation

generate -g csharp-netcore --additional-properties=targetFramework=netstandard2.0

Steps to reproduce

1. Put something in DefaultHeaders, either in GlobalConfiguration.Instance or as a Configuration constructor parameter to an API object.
2. Make any request through the generated SDK.
3. Observe that default headers are not sent with any request.

Related issues/PRs

N/A

Suggest a fix

Analyzing the generated code, it turns out there is not a single reference to the DefaultHeader property in the context of a HTTP request being constructed or sent. Clearly, the code template has to be fixed.