New warning in `as.data.frame.DataFrame()`

[Bisaloo]

(Discovered while working on const-ae/lemur#30.)

Since r-devel/r-svn@9e4249f, as.data.frame.DataFrame() will return the following warning:

Warning message:
In as.data.frame.DataFrame(x[[i]], optional = TRUE, validRN = FALSE) :
arguments in '...' ignored

because of the new validRN argument passed down from the as.data.frame() generic.

I believe the base R diff linked above makes the issue quite visible but please let me know if you'd like that I create a reprex.