title: Force pandas.to_excel to write data as 'Text' instead 'General' format
body: <p>I've already set cell format as 'Text' in the target Excel column. However pandas.to_excel changes the format to 'General' when writing strings to this column, eventually the column ends up with blank cells are formatted as 'Text' and non-blank ones as 'General'. Is there a way to write data as 'Text' instead of 'General'?</p>
  
  <pre><code>def exportData(df, dstfile, sheet):
      book = load_workbook(dstfile)
      writer = pd.ExcelWriter(dstfile, engine='openpyxl', date_format='dd/mm/yyyy', datetime_format='mm/dd/yyyy hh:mm')
      writer.book = book
      writer.sheets = dict((ws.title, ws) for ws in book.worksheets)
      df.to_excel(writer, header=False, startrow=2, index=False, sheet_name=sheet)
      writer.save()
  </code></pre>