buffer: fix backwards incompatibility

[mscdex]

4a86803 introduced a backwards incompatibility by accident and was not caught due to an existing test that wasn't strict enough.

This commit fixes both the backwards incompatibility and the test.

This changes does not affect performance.

CI: https://ci.nodejs.org/job/node-test-pull-request/7424/

/cc @Trott

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

• buffer

[cjihrig]

LGTM. Can the parens in lib/buffer.js be dropped though?

[Trott]

I'm not opposed to this, but I want to suggest the possibility that we only want this on v7.x and earlier. I'm not sure it makes a lot of sense to have two different error messages for an invalid encoding, so the accidental removal of one of them in 4a86803 arguably makes the error messages better/more consistent and we might as well do it in 8.x.x and forward.

I say "arguably" because you could make the case that the one that specifically says "string" is better, and I suspect someone might make that argument. :-D

[Trott]

LGTM if CI is green.

[mscdex]

@cjihrig I prefer to include parens when using conditionals outside of an if, while, for, etc. so that they're easier to spot visually and the intention is clear. Also it's consistent with the style of the code that was already merged.

[jasnell]

Landed in 7cd0d4f

[gibfahn]

Depends on #12361.