Add into_inner method to access inner buffer

[boudra]

This implements a into_inner method that returns the inner buffer, like BufReader does.

My use case for this was needing to seek the inner reader, I tried implementing Seek but there were complications when clearing all the internal buffers and keeping track of BOM detections, so I thought it would be easier to just expose this method and then seek the reader in my own code, which could cover other use cases where access to the inner reader is needed.