fixes passing options to writable stream

It was not possible to pass any options to the allocation of the writable stream. See http://nodejs.org/api/stream.html#stream_class_stream_writable_1 for the possible options.

This is especially useful to allocate a writable stream with a non-default highWaterMarker e.g.:

```js
var writeStream = new streams.WritableStream({
	highWaterMark: 32768
});
```