def merge_cells(cells)
      merged_cells.add cells
    end
Creates merge information for this worksheet.
Cells can be merged by calling the merge_cells method on a worksheet.
@example This would merge the three cells C1..E1    #
worksheet.merge_cells "C1:E1"
# you can also provide an array of cells to be merged
worksheet.merge_cells worksheet.rows.first.cells[(2..4)]
#alternatively you can do it from a single cell
worksheet["C1"].merge worksheet["E1"]
@param [Array, string] cells