Do not disposed linked token in HttpClient's SendAsyncWorker

If this is disposed and `HttpCompletionOption.ResponseHeadersRead` is true, there is no way to cancel reading the rest of the response body later if the `CompletionTokenSource` is disposed. This effectively prevents you from using `HttpCompletionOption.ResponseHeadersRead` to decide to cancel the network request upon seeing via the headers that you don't need the data.

Since the underlying `HttpMessageHandler` doesn't receive the `ResponseHeadersRead` and must therefore return the content stream as soon as it is available (i.e. even before the headers come in), it is quite problematic to properly implement both cancellation and returning as soon as we have a valid network stream to enable `HttpCompletionOption.ResponseHeadersRead`.