title: Why does Python petl return the following exception: "TypeError: unorderable types: str() < int()"
body: <p>While running the following code:</p>
  
  <pre><code>import petl
  data = [{'one': 'foo', 1: 'bar'}]
  table = petl.fromdicts(data)
  print(table)
  </code></pre>
  
  <p>I encountered the following exception:</p>
  
  <pre><code>...
  File "C:\Python34\lib\site-packages\petl\io\json.py", line 132, in __iter__
      hdr = sorted(hdr)
  TypeError: unorderable types: str() &lt; int()
  </code></pre>