title: python pandas: pivot_table silently drops indices with nans
body: <p>Is there an option not to drop the indices with <code>NaN</code> in them? I think silently dropping these rows from the pivot will at some point cause someone serious pain.</p>
  <pre><code>import pandas
  import numpy
  
  a = [['a', 'b', 12, 12, 12], ['a', numpy.nan, 12.3, 233., 12], ['b', 'a', 123.23, 123, 1], ['a', 'b', 1, 1, 1.]]
  
  df = pandas.DataFrame(a, columns=['a', 'b', 'c', 'd', 'e'])
  
  df_pivot = df.pivot_table(index=['a', 'b'], values=['c', 'd', 'e'], aggfunc=sum)
  print(df)
  print(df_pivot)
  </code></pre>
  <p>Output:</p>
  <pre><code>   a    b       c    d   e
  0  a    b   12.00   12  12
  1  a  NaN   12.30  233  12
  2  b    a  123.23  123   1
  3  a    b    1.00    1   1
            c    d   e
  a b                 
  a b   13.00   13  13
  b a  123.23  123   1
  </code></pre>