title: pands return '/' instead of ',' when aggregating a column
body: <p>I don't know what I did wrong to get '/' as a separator instead of ','</p>
  
  <pre class="lang-py prettyprint-override"><code>df = pd.read_html('https://en.wikipedia.org/wiki/List_of_postal_codes_of_Canada:_M')[0]
  
  #get indexes of row that contains a not assigned borough
  indexNames = df[ df['Borough'] == 'Not assigned' ].index
  
  # Delete these row indexes from dataFrame
  df.drop(indexNames , inplace=True)
  df.groupby('Postal code').agg({'Borough' : 'first', 'Neighborhood' : ','.join}).reset_index().reindex(columns=df.columns)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/15wpe.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/15wpe.png" alt="enter image description here"></a></p>