[dotnet core] [csharp-dotnet2] Add a method to edit default headers

Description

Similar to the configuration class from c#, it sould be possible to set the default headers for each HTTP request.
At the moment adding deafult header will cause duplicates, beacause there is no way to remove the previous header.

Swagger-codegen version

CodeGen version: 3

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/6976

Suggest a fix/enhancement

fix duplicate problem:

public ApiClient AddDefaultHeader(string name, string value) {
        if(defaultHeaders.ContainsKey(name))
            defaultHeaders.Remove(name);
        defaultHeaders.Add(name, value);
        return this;
    }

or add something like

public void SetDefaultHeader(HttpHeaders headers){
       defaultHeaders = headers;
}

Start by locating the generated C# ApiClient and its AddDefaultHeader implementation, then review related issue 6976. Confirm how defaultHeaders currently handles repeated names and define done as providing a way to replace an existing default header without duplicates, with coverage for the chosen behavior.