According to rstudio#418, improve buttons.html5.min.js. This builds o…

…n commit 6b56d58 and process the col/row span information to merge the cells at the excel output  via the function excelHtml5

inst/htmlwidgets/lib/datatables-extensions/Buttons/js/buttons.html5.min.js

@@ -444,16 +444,114 @@
             p.title && (r([p.title], g), k(g, /*d.header.length*/ d.header[d.header.length - 1].length - 1));
             p.messageTop && (r([p.messageTop],
                 g), k(g, /*d.header.length*/ d.header[d.header.length - 1].length - 1));
+
             /* ----- BEGIN added Code ----- */
             //c.header && (r(d.header, g), i("row:last c", e).attr("s", "2"));
 
 			if(c.header){
+				row_start_headers = g;
 				for (o = 0; o < d.header.length; o++){
 	                  r(d.header[o], g);
 	                  i("row:last c", e).attr("s", "2");
 	                }
             }
 
+            // process merged cells:
+            var mgCnt = 0;
+            var merges=[];
+            if (c.header) {
+                //for each header row
+                for(o=0; o < d.header.length; o++)
+                {
+                  //for each column (cell) in the row
+                  for(oj=0; oj<d.header[o].length; oj++)
+                  {
+                    //look for a non-colspan/rowspan cell
+                    if(d.header[o][oj] != "")
+                    {
+                      var startRow = o;
+                      var startCol = oj;
+                      var endRow = o;
+                      var endCol = oj;
+
+                      //lookahead
+                      if(oj+1 < d.header[o].length){ 
+	                      if(d.header[o][oj+1] == "") //is the cell to the right a colspan?
+	                      { 
+
+	                        startCol = oj;
+	                        endCol = oj+1;
+
+	                        //get to the last column in the colspan
+	                        while(endCol < d.header[o].length && d.header[o][endCol] == "")
+	                        {
+	                          endCol++;
+	                        }
+	                        endCol--;
+	                      }
+	                   }
+
+                      if(o+1 < d.header.length) {
+	                      if(d.header[o+1][oj] == "") //is the cell below a rowspan?
+	                      {  
+
+	                        startRow = o;
+	                        endRow = o+1;
+
+	                        //get to the last row in the rowspan
+	                        while(endRow < d.header.length && d.header[endRow][oj] == "")
+	                        {
+	                          endRow++;
+	                        }
+	                      }
+	                  }
+
+                      //create and store merge ranges
+                      //if endCol or endRow show movement
+                      if(startRow != endRow || startCol != endCol)
+                      {
+                        sC = z(startCol); //convert startCol to excel column letter
+                        sR = startRow+1+row_start_headers; // actual row to be merged needs to take into account 0 index and current row (g)
+                        eC = z(endCol); //convert endCol to excel column letter
+                        eR = endRow+row_start_headers; // actual row to be merged needs to take into account 0 index and current row (g)
+
+                        merges[mgCnt] = sC+""+sR; //start of range
+
+                        if(endCol > startCol){ //end column
+                          merges[mgCnt] = merges[mgCnt] + ":" + eC;
+                        } else {
+                          merges[mgCnt] = merges[mgCnt] + ":" + sC;
+                        }
+
+                        if(endRow > startRow) { //end row
+                          merges[mgCnt] = merges[mgCnt] + eR;
+                        } else {
+                          merges[mgCnt] = merges[mgCnt] + sR;
+                        }
+
+                        mgCnt++; //increment number of merge ranges
+                      }
+                    }
+                  }
+                }               
+            }
+
+
+            if (mgCnt > 0) {
+              //add each merge range as a child
+              var oc = i("mergeCells", e);
+              for(o=0;o<mgCnt;o++)
+              {
+
+              	oc[0].appendChild(q(e, "mergeCell", {
+                        attr: {
+                            ref: merges[o]
+                        }
+                    }));
+              	oc.attr("count", parseFloat(oc.attr("count")) + 1);
+              }
+            }
+
             /*END added code*/
             for (var o = 0, u = d.body.length; o < u; o++) r(d.body[o], g);
             c.footer && d.footer && (r(d.footer, g), i("row:last c", e).attr("s", "2"));