title: Pandas to_csv index=False not working when writing incremental chunks
body: <p>I'm writing a fixed-width file to CSV. Because the file is too large to read at once, I'm reading the file in chunks of 100000 and appending to CSV. This is working fine, however it's adding an index to the rows despite having set <code>index = False</code>.</p>
  
  <p>How can I complete the CSV file without index?</p>
  
  <pre><code>infile = filename
  outfile = outfilename
  cols = [(0,10), (12,19), (22,29), (34,41), (44,52), (54,64), (72,80), (82,106), (116,144), (145,152), (161,169), (171,181)]
  
  for chunk in pd.read_fwf(path, colspecs = col_spec, index=False, chunksize=100000):
  chunk.to_csv(outfile,mode='a')
  </code></pre>