rowsum() may return a vector instead of a matrix (PR#3737)

If all rows are in the same "group", rowsum() returns a vector instead of a
(1xN) matrix, contrary to documentation:

Thomas Lumley <tlumley@u.washington.edu> replied:

Is there a strong preference for changing the function vs changing the
documentation?

I could live with it either way, but I will point out that the "Examples"
section implies an equivalence between:
  R> xsum <- rowsum(x, group)
and
  R> xsum2 <- tapply(x, list(group[row(x)], col(x)), sum)
  R> xsum3 <- aggregate(x, list(group), sum)
which is broken by this new behavior; xsum2 and xsum3 are (1xN) matrices.

-- David Brahm (brahm@alum.mit.edu)