title: pandas: to_csv append mode with preserved columns order
body: <p>I am using:</p>
  
  <p><code>df.to_csv('file.csv', header=False, mode='a')</code></p>
  
  <p>to write multiple pandas dataframe one by one to a CSV file.</p>
  
  <p>I make sure that these dataframe have the same sets of column names.</p>
  
  <p>However, it seems that the column orders will be written in a random order, so I have a chaos CSV file.</p>
  
  <p>How to make sure that the new dataframe will be written with the column order of previous data?</p>
  
  <p>Many thanks</p>