title: Extract dtypes from dataframe and map to header
body: <p>I need to return json to front end about the dataframe's column header and its datatype. I have tried: </p>
  
  <pre><code>return {"column header" : header, "filename" : newFilename, 
          "data": result.head(n= rowsLimit).values.tolist(), 
          "datatype": result.dtypes.tolist()}
  </code></pre>
  
  <p>On <code>result.dtypes.tolist()</code>, I am getting an <code>dtype('int64') is not JSON serializable</code> error. </p>
  
  <p>Thanks</p>