Don't modify the options object

[wraugh]

I want to use split2 in pipelines where I can pass each transform the
same options object, e.g.

class MyTransform extends Transform {
    _transform (chunk, encoding, callback) {
    // ...
    }
}
var options = { highWaterMark: 1024 }
pump(split2(options), new MyTransform(options), process.stdout)

If split2 adds transform and flush properties to options, then
MyTransform will also set those options, and it won't behave as
expected.

To avoid this, we can use a shallow copy of options instead of
operating on the object directly.