title: Can't center horizontally 'float' cells with xlsxwriter
body: <p>I am trying to save my pandas dataframe as excel file with <code>df.to_excel()</code> using <code>xlsxwriter</code> engine. I am trying to format header and rows with two different formats:</p>
  <pre class="lang-py prettyprint-override"><code>
  workbook = writer.book
  header_format = workbook.add_format({
     'bold' : True,
     'valign' : 'vcenter',
     'align' : 'center',
     'bg_color' : '#D7E4BC',
     'border' : 1
  })
  
  cell_format = workbook.add_format({
     'valign' :  'center',
     'align' : 'center',
     'border' : 1
  })
  
  #Add header formatting
  for col_num, value in enumerate(df.columns.values):
     worksheet.write(0, col_num + 1, value, header_format)
  
  #Add row formatting
  worksheet.conditional_formatting(1, 0, df.shape[0], df.shape[1], {'type' : 'no_blanks', 'format' : cell_format})
  </code></pre>
  <p>I don't know why, but only cells with float values aren't being centered horizontally. I would like to know how to center them too (if possible).</p>
  <p>Thanks</p>