data.table() should ignore/warning on `stringsAsFactors` argument

[jangorecki]

It would avoid the case to create stringsAsFactors recycled to length of other arguments. Like this: http://stackoverflow.com/q/33947148/2490497
Better consistency with data.frame.
Should be pretty simple and safe.

[arunsrinivasan]

I think it'd be better to implement it. The code is there in as_factor() function within fread.R.. in case you'd like to have a look at it. We just need to pull it out and use the same logic in fread() here..

[arunsrinivasan]

Thanks, but I've implemented it similar to fread() now.