[System]: Fix WebClient.UploadValuesTaskAsync(); bug #20359.
[System]: WebClient.OpenWrite() must get the response on close; fixes #10163.

When the stream returned by WebClient.OpenWrite() / OpenWriteAsync() /
OpenWriteTaskAsync(), we need to call GetResponse() and read the full
response.

Unfortunately, there is no async API available for this, so this API should
not be used in async applications.