title: How to add new column with handling nan value
body: <p>I have a dataframe like this</p>
  
  <pre><code>    A   B
  0   a   1
  1   b   2
  2   c   3
  3   d  nan
  4   e  nan
  </code></pre>
  
  <p>I would like to add column C like below</p>
  
  <pre><code>    A   B    C
  0   a   1    a1
  1   b   2    b2
  2   c   3    c3
  3   d  nan   d
  4   e  nan   e
  </code></pre>
  
  <p>So I tried </p>
  
  <pre><code>df["C"]=df.A+df.B
  </code></pre>
  
  <p>but It returns </p>
  
  <pre><code> C
  a1
  b2
  c3
  nan
  nan
  </code></pre>
  
  <p>How can get correct result?</p>