removed stream-cache, changed to use native readable stream

hapi@8.0.0-rc9 implemented a [check](https://github.com/hapijs/hapi/commit/98d41213efc2e8fb123b5a52701fccf9da9f3b82#diff-d86a59ede7d999db4b7bc43cb25a1c11) on response for stream to make sure that the stream is not in `objectMode`. The stream-cache object doesn't inherit from stream.Readable, so it has no `_readableState` property. I'm not sure why a stream cache was used in this place. Maybe i missed something.