title: error `unhashable type: 'Index'` on attempting to standardize columns in Pandas DataFrame with DatetimeIndex
body: <p>I have a pandas DataFrame with a DatetimeIndex and columns that I want to standardize (rescale to between -1 and 1). When I attempt to do this, I encounter the error <code>unhashable type: 'Index'</code>. What might be going wrong and how might I proceed?</p>
  
  <pre><code>from sklearn.preprocessing import MinMaxScaler
  scaler = MinMaxScaler()
  
  df[[df.columns]] = scaler.fit_transform(df[[df.columns]])
  </code></pre>