Stream.Flush should reuse buffer

[robfig]

Stream.Flush copies the contents of the stream's buffer to the output, but instead of resetting its buffer to reuse the allocation, it keeps appending to the end of the buffer. For large JSON objects this results in much more allocation than necessary.

// Flush writes any buffered data to the underlying io.Writer.
func (stream *Stream) Flush() error {
	if stream.out == nil {
		return nil
	}
	if stream.Error != nil {
		return stream.Error
	}
	n, err := stream.out.Write(stream.buf)
	if err != nil {
		if stream.Error == nil {
			stream.Error = err
		}
		return err
	}
	stream.buf = stream.buf[n:]  // <--- HERE
	return nil
}

I think the marked line above should instead be

	stream.buf = stream.buf[:0]

Am I missing something?

[robfig]

I've opened a pull request to make this update and add a benchmark which demonstrates that allocations are made prior to the change, but not after. Please see #440