Fix _buffer_encode() wrong return type.

[elliotwutingfeng]

Current implementation of _buffer_encode() has the wrong type signature (expected bytes but got str). It also attempts to perform a str.join() on list[bytes].

Fixes #138

[jribbens]

Hmm it seems rather unimpressive that mypy does not spot that str.join(list[bytes]) will fail.

There is a wider issue here. The codec decoder also doesn't work properly. The existing tests in test_idna_codec.py should be noticing both of these issues, except that, even after import idna.codec, codecs.lookup('idna') is still returning the Python standard library codec instead of the one from this library, so the tests are just testing the Python codec instead of this library's one.

[sethmlarson]

I noted here that the codec implementation appears to be untested, broken, and likely unused. #134

[elliotwutingfeng]

I see. I think further discussion would be needed before attempting to fix this.

[jribbens]

@elliotwutingfeng I think your fix was fine, I have incorporated it into my PR #145 which also fixes the other issues.

[elliotwutingfeng]

Thanks! Yes, it is better to consolidate all the fixes into one PR.