title: pandas to_csv converts str column to int(or float)
body: <p>As untitled, I noticed that pandas 'to_csv' transforms automatically columns where there are only alphanumerical strings to  float .
  I am creating a dataframe in Jupyter notebook and creating a column ['A'] full of values '1'. Hence, I have a dataframe composed of a column of string '1'.
  When i convert my dataframe to csv file with 'to_csv'. the output csv file is a one column full of integers 1.<br>
  You may advise me to reconvert the column to string when reloaded in jupyter, However that's won't work because I don't know beforehand what columns may be penalized because of this behaviour. 
  Is there a way to avoid this strange situation.</p>