title: Reducing pandas series with multiple nan values to a set gives multiple nan values
body: <p>I'm expecting to get <code>set([nan,0,1])</code> but I get <code>set([nan, 0.0, nan, 1.0])</code>:</p>
  
  <pre><code>&gt;&gt;&gt; import numpy as np
  &gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; l= [np.nan,0,1,np.nan]
  &gt;&gt;&gt; set(pd.Series(l))
  set([nan, 0.0, nan, 1.0])
  &gt;&gt;&gt; set(pd.Series(l).tolist())
  set([nan, 0.0, nan, 1.0])
  &gt;&gt;&gt; set(l)
  set([nan, 0, 1])
  </code></pre>