Clone sheetFront to fix width of columns when export is called many times

[YoannB]

In Sheet.js, the var "sheetFront" was used with operator "+=", so each time we call execute with columns config, "sheetFront" is modified.

Can you try to clone "sheetFront" when generate is called :

Sheet.prototype.generate = function(){
    var sheetFrontClone = sheetFront.slice(0);

    //....

    if (colsWidth !== "") {
        sheetFrontClone += '<cols>' + colsWidth + '</cols>';
    }
    xlsx.file(config.fileName, sheetFrontClone + '<x:sheetData>' + rows + '</x:sheetData>' + sheetBack);
}

[YoannB]

I do many fixes in a fork, if you want see :

https://github.com/YoannB/Node-Excel-Export