many .date methods return "any" type

From your example:

```ts
import LuxonAdapter from '@date-io/luxon'
import DateFnsAdapter from '@date-io/date-fns'

const dateFns = new DateFnsAdapter()
const luxon = new LuxonAdapter({ locale: 'fr' }) // pass french locale

const initialLuxonDate = luxon.date('2018-10-28T11:44:00.000Z')
const initialDateFnsDate = dateFns.date('2018-10-28T11:44:00.000Z')

const updatedLuxonDate = luxon.addDays(initialLuxonDate, 2)
const updatedDateFnsDate = dateFns.addDays(initialDateFnsDate, 2)

luxon.format(updatedLuxonDate, 'fullDateTime24h') // "2018, octobre 30 11:44"
dateFns.format(updatedLuxonDate, 'fullDateTime24h') // "2018, October 30th 11:44"
```

* `luxon.date`: returns `any`
* `dateFns.date`: returns `Date`

I also checked `moment` and it also returns `any`.