title: writeFile not woking in exceljs
body: <p>i am using <a href="https://github.com/guyonroche/exceljs" rel="nofollow noreferrer">exceljs</a> and try to write value in a cell but it does not working. However <code>workbook.xlsx.readFile(filename)</code> is working but <code>workbook.xlsx.writeFile(filename)</code> won't. </p>
  
  <p>Here is my code:-</p>
  
  <pre><code>var Excel = require('exceljs');
  var fs = require('fs')
  module.exports.summary = function(req, res, next) {
  try {
      var filename = process.cwd() + '/template/report/summary.xlsx';
      var workbook = new Excel.Workbook();
      workbook.xlsx.writeFile(filename)
          .then(function() {
              var worksheet = workbook.getWorksheet(1);
              console.log('worksheet',worksheet);
              var row = worksheet.getRow(5);
              row.getCell('C').value = new Date();
              row.commit();
              worksheet.commit();
  
              workbook.commit().then(function() {
                  console.log('xls file is written.');
              });
              res.json({ msg: done })
          });
  
   } catch (e) {
      next(e);
   }
  }
  </code></pre>