[Rd] cumprod doesn't work with data frames (PR#2667)

[jsisk1 at pacbell.net]

Full_Name: J. Sisk
Version: 1.6.1
OS: Linux (RedHat 8)
Submission from: (NULL) (67.119.41.66)


Suppose you make a data-frame like so:

    xxx <- data.frame(a=10,b=20,c=30,d=40)

Then 

    cumprod(xxx[1,]) 

returns 

> cumprod(xxx[1,])
   a  b  c  d
1 10 20 30 40

The documentation for cumprod says that it should work on "numerical objects",
and this is a data-frame, but it seems like it ought to "do the right thing" in
this case.  If this is redundant, my apologies.