Summary

Closes#1341

Test plan

I confirmed the type isRange['range'][]:

• exceljs/lib/doc/worksheet.js

  Lines 877 to 881 inec92cb3

  	// Merges
  	model.merges=[];
  	_.each(this._merges,merge=>{
  	model.merges.push(merge.range);
  	});

• exceljs/lib/doc/worksheet.js

  Lines 612 to 632 inec92cb3

  	_mergeCellsInternal(dimensions,ignoreStyle){
  	// check cells aren't already merged
  	_.each(this._merges,merge=>{
  	if(merge.intersects(dimensions)){
  	thrownewError('Cannot merge already merged cells');
  	}
  	});
  	// apply merge
  	constmaster=this.getCell(dimensions.top,dimensions.left);
  	for(leti=dimensions.top;i<=dimensions.bottom;i++){
  	for(letj=dimensions.left;j<=dimensions.right;j++){
  	// merge all but the master cell
  	if(i>dimensions.top||j>dimensions.left){
  	this.getCell(i,j).merge(master,ignoreStyle);
  	}
  	}
  	}
  	// index merge
  	this._merges[master.address]=dimensions;

• exceljs/lib/doc/worksheet.js

  Lines 608 to 609 inec92cb3

  	constdimensions=newRange(cells);
  	this._mergeCellsInternal(dimensions,true);