title: pandas checking for nan not working using .isin()
body: <p>I have the following pandas Dataframe with a <code>NaN</code> in it. </p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame([1,2,3,float('nan')], columns=['A'])
  df
  
      A
  0   1
  1   2
  2   3
  3 NaN
  </code></pre>
  
  <p>I also have the list <code>filter_list</code> using which I want to filter my Dataframe. But if i use <code>.isin()</code> function, it is not detecting the <code>NaN</code>. Instead of getting <code>True</code> I am getting <code>False</code> in the last row</p>
  
  <pre><code>filter_list = [1, float('nan')]
  
  df['A'].isin(filter_list)
  0     True
  1    False
  2    False
  3    False
  Name: A, dtype: bool
  </code></pre>
  
  <p>Expected output:</p>
  
  <pre><code>0     True
  1    False
  2    False
  3    True
  Name: A, dtype: bool
  </code></pre>
  
  <p>I know that I can use <code>.isnull()</code> to check for <code>NaNs</code>. But here I have other values to check as well. I am using pandas <code>0.16.0</code> version</p>
  
  <p>Edit: The list <code>filter_list</code> comes from the user. So it might or might not have <code>NaN</code>. Thats why i am using <code>.isin()</code></p>