Merging Some of Table Cells Using tableGrob

Question

I was wondering how to merge some of the header cells. Here is the example:

# example data & header row
tab <- tableGrob(mtcars[1:3, 1:3], rows=NULL)
header <- tableGrob(mtcars[1, 1:2], rows=NULL, cols=c("head1", "head2")) 

jn <- gtable_combine(header[1,], tab, along=2)
jn$widths <- rep(max(jn$widths), length(jn$widths)) # make column widths equal
grid.newpage()
grid.draw(jn) # see what it looks like before altering gtabletype here

I am expecting table like this:

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