title: Column order in pandas.concat
body: <p>I do as below:</p>
  
  <pre><code>data1 = pd.DataFrame({ 'b' : [1, 1, 1], 'a' : [2, 2, 2]})
  data2 = pd.DataFrame({ 'b' : [1, 1, 1], 'a' : [2, 2, 2]})
  frames = [data1, data2]
  data = pd.concat(frames)
  data
  
  
     a    b
  0   2   1
  1   2   1
  2   2   1
  0   2   1
  1   2   1
  2   2   1
  </code></pre>
  
  <p>The data column order is in alphabet order. Why is it so?
  and how to keep the original order?</p>