title: Excel::Writer::XLSX set_optimization() only writes numeric data
body: <p>I am using the Excel::Writer::XLSX perl module to export to Excel. It is working great except large exports take a long time. However, when I add $workbook->set_optimization(); I only get numeric data in my .xlsx file.</p>
  
  <p>I have tried using both $worksheet->write() and $worksheet->write_string() but any fields that have a string rather than a number are being left out of the export when I use $workbook->set_optimization();</p>
  
  <p>If I use the older Spreadsheet::WriteExcel module it works just fine but the file size of the export is substantially larger, 24MB vs. 7MB</p>
  
  <p>Here is the code for creating the header row:</p>
  
  <pre><code># Create a new workbook
  my $workbook = Excel::Writer::XLSX-&gt;new($file_path);
  
  $workbook-&gt;set_optimization();
  
  my $worksheet = $workbook-&gt;add_worksheet();
  # insert the header row
  my $header_cnt = 0;
  foreach my $header_cell (@header_values){#loop through each header column and add it to the sheet
      # print $header_cell." - ".$header_cnt."\n";
      # $worksheet-&gt;write_string("0", $header_cnt, "test");
      $worksheet-&gt;write_string("0", $header_cnt, $header_cell);
      $header_cnt++;
  }
  
  $workbook-&gt;close() or die "Error closing file: $!";
  </code></pre>