null_io.rb - add set_encoding, similar to IO#set_encoding

[MSP-Greg]

Description

See https://github.com/rack/rack/pull/2115, which identifies a bug that occurs with Rack::MockRequest.env_for.

See also Rack Spec: The Input Stream.

More IO instance methods may need to be added to Puma::NullIO.

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[MSP-Greg]

@nateberkopec Thanks.

The current PR adds set_encoding, I think adding external_encoding and internal_encoding would also be a good idea. I'll update soon...