I am trying to Shim the following code:

```
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
request.Method = "GET";
request.Headers.Add("Authorization", "Bearer " + authToken.token.access_token);
request.Accept = "application/json";

```

But running the Unit Test throws an exception in this part: `request.Headers.Add()` because `request.Headers` is `null`. This, in spite of initializing `Headers` in my test:

```
ShimHttpWebRequest request = new ShimHttpWebRequest();
ShimWebRequest.CreateString = (urio) => {
   request.Instance.Headers = new WebHeaderCollection {
        {"Authorization", "Bearer abcd1234"}
   };
   //also tried initilizing it like this:
   //WebHeaderCollection headers = new WebHeaderCollection();
   //headers[HttpRequestHeader.Authorization] = "Bearer abcd1234";
   //request.Instance.Headers = headers;
   return request.Instance;
};

```

But `request.Instance.Headers` is still `null`.

What am I missing?