Cookie header is not passed by C# client

Description

When generating the Client it is not possible to specify the Cookie header and have it passed through to the server.

Swagger-codegen version

2.2.1

Command line used for generation

swagger-codegen-cli-2.2.1.jar generate -l csharp

Steps to reproduce

Generate a swagger definition as C#
Add "Cookie" to the defaultheader
Observe that the request sent does not contain the cookie.

Suggest a fix/enhancement

By updating the ApiClient.cs file I was able to pull out the Cookie header and assign it to the CookieContainer utilized by the RestSharp library.

            // set timeout
            RestClient.Timeout = Configuration.Timeout;
            // set user agent
            RestClient.UserAgent = Configuration.UserAgent;
            // set cookies
            var cookies = new CookieContainer();
            foreach (var c in headerParams.Where(x => x.Key.Equals("Cookie", StringComparison.CurrentCultureIgnoreCase)).First().Value.Split(new string[] { "; " }, StringSplitOptions.None))
            {
                var k = c.Split('=');
                if (k.Length < 2) continue;
                cookies.Add(new Uri(RestClient.BaseUrl.AbsoluteUri), new Cookie(k[0], k[1]));
            }
            RestClient.CookieContainer = cookies;

https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache#L206

Start in modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache at the referenced client setup around line 206. Generate a C# client from a Swagger definition with Cookie in the default headers, then verify that the outgoing request contains the cookie; the existing reproduction and suggested CookieContainer approach define the expected behavior.