get_pos does not seem to account for the current speed

The [get_pos](https://docs.rs/rodio/latest/rodio/struct.Sink.html#method.get_pos) function does not seem to account for speed, so slowing the song down to, for example, `0.10` using [set_speed](https://docs.rs/rodio/latest/rodio/struct.Sink.html#method.set_speed) the speed increases as if it was still `1.00` speed.