Default method needed for as_array()

[ralmond]

I generally expect as_XXX functions to be harmless if given an object of type XXX, but

> as_array(matrix(1:6,2,3))
Error in UseMethod("as_array", x) : 
  no applicable method for 'as_array' applied to an object of class "c('matrix', 'array', 'integer', 'numeric')"

The function base::as.array() does seem to have the default behavior.

May I suggest adding

as_array.default <- function (x) base::as.array.default(x)

To give the expected behavior.

[dfalbel]

@ralmond I agree! Do you want to send a PR with this change?