anchor.diff:  let internal addHeader (client:HttpClient) (headerKey:string) (headerVal:string) =
       client.DefaultRequestHeaders.Add(headerKey, headerVal)
   
  -let useDefaultHandler = 
  +let useDefaultHandler =
       match Environment.GetEnvironmentVariable("PAKET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER") with
       | null -> false
       | env ->
  
positive.code: 
          let client = new HttpClient(handler)
          match auth with
          | None -> handler.UseDefaultCredentials <- true
          | Some(Credentials({Username = username; Password = password; Type = AuthType.Basic})) ->
              // http://stackoverflow.com/questions/16044313/webclient-httpwebrequest-with-basic-authentication-returns-404-not-found-for-v/26016919#26016919
  
negative.code: 
          let client = new HttpClient(handler)
          match auth with
          | None when not (isNull githubToken) ->
              client.DefaultRequestHeaders.Authorization <-
                  new System.Net.Http.Headers.AuthenticationHeaderValue("token", githubToken)
          | None -> handler.ServerCredentials <- CredentialCache.DefaultCredentials
          | Some(Credentials({Username = username; Password = password; Type = AuthType.Basic})) ->
              // see lengthy comment below.
  