Fix line-ending backslash whitespace escaping for multiline strings

[cespare]

Fixes #372.

[arp242]

It would be better to put the test for this in toml-test, rather than adding a new TestDecodeMultilineStringBackslash for this – that way other TOML implementations benefit from this too, and it's also easier.

There's a copy of that in internal here, and it gets tested with just go test (no need to run the toml-test binary like before) – can just modify that and I'll sync toml-test with it at some point.

Specifically: https://github.com/BurntSushi/toml/blob/master/internal/toml-test/tests/valid/string/multiline.toml (and the corresponding .json file).

[cespare]

can just modify that and I'll sync toml-test with it at some point.

Interesting -- this repo is the source of truth for toml-test? I had assumed it was the other way around.

[arp242]

Interesting -- this repo is the source of truth for toml-test? I had assumed it was the other way around.

It is; I just didn't want to bother you with having to create two PRs.

[cespare]

OK, thanks! Done.