first week of 2019 issue

[elsafy]

Description of the Issue and Steps to Reproduce:
There is an issue which only happens for the first week of 2019, because it starts on 31 Dec 2018, please check the code below to reproduce
Environment:

Chrome 69 on OSX

const week = "2019-W01";
const weekResult = moment(week).format(moment.HTML5_FMT.WEEK);
console.log(weekResult);
// expected result: 2019-W01
// actual result: 2018-W01

[marwahaha]

@ashsearle does #4700 address this?

[elsafy]

@marwahaha yep definitely, thanks a lot for the hint

[ashsearle]

@marwahaha Yes. I don't know why that PR failed to go green, but I'll happily do the close/re-open dance to get a green light once your PR fixing the build is merged in.