NSwag generated C# clients fail to set Content-Language header as it must be set on Content headers

[kmgallahan]

NSwag generated code from openapi specifications that indicate the Content-Language header is required for a POST request tries to do the following:

request_.Headers.TryAddWithoutValidation("Content-Language", ConvertToString(content_Language, System.Globalization.CultureInfo.InvariantCulture));

Which always fails because Content-Language is not a valid header there. Rather, it must be added to the content headers:

request_.Content = content_;
request_.Content.Headers.TryAddWithoutValidation("Content-Language", ConvertToString(content_Language, System.Globalization.CultureInfo.InvariantCulture));