regularize date function only respects first possible argument

[mbannert]

because class(x)[1] "Date" class is not respected, if it's not POSIXct – it's not working.

Error in regularize_date(x$time) : 
  class(x)[1] %in% c("POSIXct", "Date") is not TRUE

This causes problems when using the popular fread function from data.table because they return c("IDate","Date") none of which is POSIXct. I believe one should definitely use inherits here instead of class.

Fill provide PR fix.