title: Python openpyxl fails to save existing excel having customized column width
body: <p>I'm generating one Excel with a template,</p>
  
  <pre><code>from openpyxl import Workbook
  from openpyxl import load_workbook
  
  src_file = 'D:\\code\\TestLink-Report\\pyxl.xlsx'
  wb = load_workbook(filename = src_file, guess_types = False)
  
  sheet = wb.worksheets[0]
  sheet.cell('A1').value = 'blabla'
  wb.save(filename =  src_file)
  </code></pre>
  
  <p>If src_file is new blank one, it works.
  If I change width of any column in src file by manual, run code again, it fails</p>
  
  <pre><code>    wb.save(filename =  src_file)
    File "D:\Python27\envs\TestLink\lib\site-packages\openpyxl\workbook.py", line 265, in save
      save_workbook(self, filename)
    File "D:\Python27\envs\TestLink\lib\site-packages\openpyxl\writer\excel.py", line
   187, in save_workbook
      writer.save(filename)
    File "D:\Python27\envs\TestLink\lib\site-packages\openpyxl\writer\excel.py", line
   170, in save
      self.write_data(archive)
    File "D:\Python27\envs\TestLink\lib\site-packages\openpyxl\writer\excel.py", line
   98, in write_data
      self._write_worksheets(archive, shared_string_table, self.style_writer)
    File "D:\Python27\envs\TestLink\lib\site-packages\openpyxl\writer\excel.py", line
   128, in _write_worksheets
      style_writer.get_style_by_hash()))
    File "D:\Python27\envs\TestLink\lib\site-packages\openpyxl\writer\worksheet.py", line 98, in write_worksheet
      write_worksheet_cols(doc, worksheet, style_table)
    File "D:\Python27\envs\TestLink\lib\site-packages\openpyxl\writer\worksheet.py", line 209, in write_worksheet_cols
      col_def['style'] = str(style_table[hash(columndimension.style_index)])
  KeyError: 549131827
  </code></pre>
  
  <p>I'm using Excel 2013.
  I heard openpyxl doesn't support style well after searching on stackoverflow. But my problem seems that it should be one simple thing.</p>