title: Converting a column within pandas dataframe from int to string
body: <p>I have a dataframe in pandas with mixed int and str data columns. I want to concatenate first the columns within the dataframe. To do that I have to convert an <code>int</code> column to <code>str</code>. 
  I've tried to do as follows:</p>
  
  <pre><code>mtrx['X.3'] = mtrx.to_string(columns = ['X.3'])
  </code></pre>
  
  <p>or </p>
  
  <pre><code>mtrx['X.3'] = mtrx['X.3'].astype(str)
  </code></pre>
  
  <p>but in both cases it's not working and I'm getting an error saying "cannot concatenate 'str' and 'int' objects". Concatenating two <code>str</code> columns is working perfectly fine.</p>