title: Python to_csv the missing 0 in front of zipcode
body: <p>I have a data frame </p>
  
  <p>USER = </p>
  
  <pre><code>   zipcode  userCount
  0   00601   5
  1   00602   23
  2   00603   53
  3   00604   2
  4   00605   6
  5   00606   10
  6   00610   8
  7   00612   33
  8   00613   2
  9   00614   2
  10  00616   1
  11  00617   9
  12  00622   6
  13  00623   28
  14  00624   10
  15  00627   8
  16  00631   1
  17  00637   13
  18  00638   9
  19  00641   12
  20  00646   13
  </code></pre>
  
  <p>When I save it </p>
  
  <pre><code>USER.to_csv('Total_user.csv',index = False)
  </code></pre>
  
  <p>I got missing 0 in front of the zipcode. 00601 -> 601</p>
  
  <pre><code>zipcode userCount
  601 5
  602 23
  603 53
  604 2
  605 6
  606 10
  610 8
  612 33
  613 2
  614 2
  616 1
  617 9
  622 6
  623 28
  624 10
  627 8
  631 1
  637 13
  638 9
  641 12
  646 13
  </code></pre>
  
  <p>Is that anything I missed in the to_csv line? I just want to save the 0 in front of the csv. Then when I read_csv(low_memory = False) Then the zipcode has the normal format. </p>