R-2.2.0 format.default() loses matrix structure (PR#8696)

If argument big.mark is supplied.  Like so:

version         _

platform i686-redhat-linux-gnu
arch     i686
os       linux-gnu
system   i686, linux-gnu
status
major    2
minor    2.0
year     2005
month    10
day      06
svn rev  35749
language R

mat <- matrix(1:6, 3)
mat

[,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6

format(mat)

[,1] [,2]
[1,] "1"  "4"
[2,] "2"  "5"
[3,] "3"  "6"

format(mat, big.mark = ",")

[1] "1" "2" "3" "4" "5" "6"