changed a way to check if .expr is function

[everpeace]

changed a way to check if expr is function in such a way that .expr
will not be evaluated.

#158

[crowding]

This checks for invocations of the form r_ply(n, function() expr) but I would have guessed that the intention of r_ply's DWIM was to catch invocations of the form

fn <- function() {...}
r_ply(n, fn)

which now generate an error with this patch.

We probably also want to maintain consistency between r_ply rlply raply and rdply.

[everpeace]

Thanks for pointing out a bug of my code.
I haven't noticed there are several places which use this check.

I closed this pull request for now.
I would open another pull request if I could make modifications for this issue.