Set Content-Length header for UnityWebRequest POST requests in 2017.3?

I am trying to send multipart form data via a UnityWebRequest to a REST service. The request is built with the following code:

 public IEnumerator Post(string uri, WWWForm data, Action<UnityWebRequest, Exception> onFinished = null)
 {
     Request = UnityWebRequest.Post(uri, data);

     yield return sendRequest();

     if (onFinished != null)
        onFinished(Request, UploadException);
 }

(This is missing some context from the rest of the class, but the important part is the UniytWebRequest.Post(uri, data); line).

In previous versions of Unity this has worked fine, but in 2017.3, the remote server has started returning a 411 Length Required error.

Looking at the request object before it is sent, I see that the Content-Type header is set, but no Content-Length header is present. I attempted to set the header manually, but got the following error: InvalidOperationException: Cannot override system-specified headers

Is there another way to set the outgoing content length header in 2017.3?