Deno and Servo fail at big5/shift_jis/euc-kr streaming

[ChALkeR]

See servo/servo#41259, denoland/deno#31605

I haven't tested encoding_rs manually, but given that both Deno and Servo use it, it's a likely cause

Compact testcase: ExodusOSS/bytes@4b758ba

[ChALkeR]

This seems to happen only on zero-length streaming chunks.
Those should not change the internal state, but they do

Per spec, there is nothing to handle in them at all -- no input bytes in queue, no EOF.
The decoder instance (or its state) should not be touched.

Currently, decode(a, { stream: true }) + decode(b) is not equal to decode(a + b), which is troublesome, as the decoding result now depends on the shape of chunks, not just their byte contents.

And decoder result could be e.g. affected by network conditions without affecting TLS (as the bytes are still correct).

Not entirely sure if that's applicable to zero-sized chunks though!