title: Concatenate column values in Pandas DataFrame with "NaN" values
body: <p>I'm trying to concatenate Pandas <code>DataFrame</code> columns with NaN values.</p>
  
  <pre><code>In [96]:df = pd.DataFrame({'col1' : ["1","1","2","2","3","3"],
                  'col2'  : ["p1","p2","p1",np.nan,"p2",np.nan], 'col3' : ["A","B","C","D","E","F"]})
  
  In [97]: df
  Out[97]: 
    col1 col2 col3
  0    1   p1    A
  1    1   p2    B
  2    2   p1    C
  3    2  NaN    D
  4    3   p2    E
  5    3  NaN    F
  
  In [98]: df['concatenated'] = df['col2'] +','+ df['col3']
  In [99]: df
  Out[99]: 
    col1 col2 col3 concatenated
  0    1   p1    A         p1,A
  1    1   p2    B         p2,B
  2    2   p1    C         p1,C
  3    2  NaN    D          NaN
  4    3   p2    E         p2,E
  5    3  NaN    F          NaN
  </code></pre>
  
  <p>Instead of 'NaN' values in "concatenated" column, I want to get "D" and "F" respectively for this example?</p>