Strict-Transport-Security: Remove setIf

[EvanHahn]

setIf has been deprecated and will be removed in hsts@3 and helmet@4.

Luckily, the transition is relatively painless—simply do the conditional yourself.

Old:

app.use(hsts({
  maxAge: 123,
  setIf(req, res) {
    return req.secure
  }
}))

New:

const hstsMiddleware = hsts({
  maxAge: 123
})

app.use((req, res, next) {
  if (req.secure) {
    hstsMiddleware(req, res, next)
  } else {
    next()
  }
})

[EvanHahn]

Addressed in 788d69b. To be released in version 4 (see #197).