title: Writing pandas DataFrame to JSON in unicode
body: <p>I'm trying to write a pandas DataFrame containing unicode to json, but the built in <code>.to_json</code> function escapes the characters. How do I fix this?</p>
  
  <p>Example:</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame([['τ', 'a', 1], ['π', 'b', 2]])
  df.to_json('df.json')
  </code></pre>
  
  <p>This gives:</p>
  
  <pre><code>{"0":{"0":"\u03c4","1":"\u03c0"},"1":{"0":"a","1":"b"},"2":{"0":1,"1":2}}
  </code></pre>
  
  <p>Which differs from the desired result:</p>
  
  <pre><code>{"0":{"0":"τ","1":"π"},"1":{"0":"a","1":"b"},"2":{"0":1,"1":2}}
  </code></pre>
  
  <p><hr>
  I have tried adding the <code>force_ascii=False</code> argument:</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame([['τ', 'a', 1], ['π', 'b', 2]])
  df.to_json('df.json', force_ascii=False)
  </code></pre>
  
  <p>But this gives the following error:</p>
  
  <pre><code>UnicodeEncodeError: 'charmap' codec can't encode character '\u03c4' in position 11: character maps to &lt;undefined&gt;
  </code></pre>
  
  <p><hr>
  I'm using WinPython 3.4.4.2 64bit with pandas 0.18.0</p>