Add a method to unset encoding on a stream

### What is the problem this feature will solve? Currently it is impossible to unset the encoding of a `ReadableStream` once `setEncoding` is called. This makes it impossible to obtain `Buffer`s from the stream again after temporarily switching to an encoding. I know that this could be worked around but there is a much better fix which is backward compatible. In my opinion, this is a major parity issue in the API. ### What is the feature you are proposing to solve the problem? I propose adding a new special `buffer` encoding which will unset/reset the encoding so that the stream returns a buffer again. ### What alternatives have you considered? I originally thought `binary` encoding was supposed to do this but then I found out that it's just an alias for `latin1`! Any alternatives outside of the standard library will always be inferior to a new option which would be available to everyone using new versions with this proposed fix :smile: