title: HighCharts exporting filename
body: <p>I am using highcharts exporting module to export charts as pdf. In my code I have created a snippet <code>chart</code> object that gets manipulated on different gui controls being used. It looks something like this:</p>
  
  <pre><code>options = {
     ...
     ...
     exporting:{
        type: "application/pdf",
        filename: "default",
  
        buttons:{
          exportButton:{
              menuItems: null,
              onclick:function(){
                  var fileName = "AAAA";//it's dynamic in reality.
                  alert(options.exporting.filename);//alerts "default"
                  options.exporting.filename = fileName;
                  alert(options.exporting.filename);//alerts "AAAA"
                  this.exportChart();
               }
           },
           printButton: {
               enabled: false
           }
        }
     }
  }
  </code></pre>
  
  <p>Now whenever the export button is clicked the downloaded file is named as <code>default.pdf</code> while the alerts show that the property has been changed.</p>
  
  <p>Also since the first alert shows the result as <code>default</code>(which is not the default value, its <code>chart</code> actually) its clear that I am referencing the correct property, so no chance that of error due to setting filename in a wrong property.</p>
  
  <p>Can anyone explain the situation or suggest changes to make me download the file with dynamic names.?               </p>