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 Buffers 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 😄

Start by locating the ReadableStream setEncoding implementation and its existing encoding-related tests. Trace how encoding changes affect returned chunks, then define completion as supporting the proposed special buffer encoding so the stream returns Buffers again after an encoding was set.