[Rd] by should use match.fun

[Gabor Grothendieck]

I noticed that, unlike similar functions, 'by' does not use match.fun, e.g.

> by(iris, iris$Species, "summary")
Error in FUN(X[[1]], ...) : couldn't find function "FUN"

The 'by' code should have an initial line of:

FUN <- match.fun(FUN)