Fix merging of 3+ cells vertically

src/file/table/table-column.ts

@@ -15,7 +15,10 @@ export class TableColumn {
 
     public mergeCells(startIndex: number, endIndex: number): TableCell {
         this.cells[startIndex].addVerticalMerge(VMergeType.RESTART);
-        this.cells[endIndex].addVerticalMerge(VMergeType.CONTINUE);
+
+        for (let i = startIndex; i <= endIndex; i++) {
+            this.cells[i].addVerticalMerge(VMergeType.CONTINUE);
+        }
 
         return this.cells[startIndex];
     }