apirequest/isReadableStream rework

[viktorasm]

I tried using a http.get response object directly as GCS media.body, which should be a valid readable stream.

Fixes #334

[ryanseys]

What part of that check is false? I just tried it with a raw http.IncomingMessage and seemed to work fine:

> function isReadableStream(obj) { return obj instanceof stream.Stream && typeof obj._read === 'function' && typeof obj._readableState === 'object'; }
undefined
> var x = new http.IncomingMessage();
undefined
> isReadableStream(x);
true

[ryanseys]

Closing as I don't see a need for this.