Non-finite dates plus seconds / minutes / hours should yield datetime

[keesdeschepper]

See #828 (comment).
A non-finite date plus e.g. seconds now yields an object of class Date:

class(as_date(NA) + seconds(1))
#> "Date"

It makes more sense if this yields an object of class POSIXct.

[vspinu]

This is internally the same as #831 , but ok.

[vspinu]

forgot to close this one