Apostrophes not being stripped from heading anchors

I would expect a heading of `## We're on fire` to have an anchor name of `#we-re-on-fire`, but in fact it is `#We%u2019re-on-fire`. To add an extra layer of confusion - if you copy the link it comes up as `#We%E2%80%99re-on-fire`.

I have got used to the capitalisation of the first letter of the anchor (unusual in markdown), but I believe that any consecutive non-word characters should be replaced with a single hyphen.