title: NaNs not recognized in df.loc or for loops
body: <p>I currently have a df with a column <code>Outliers</code>. When I do:</p>
  <pre><code>df.Outliers.value_counts(dropna = False)
  </code></pre>
  <p>I get:</p>
  <pre><code>NaN    2862
  1.0     600
  0.0     257
  </code></pre>
  <p>However, when I try to display only these rows with:</p>
  <pre><code>df.loc[df.Outliers == np.nan] # numpy was imported as np
  </code></pre>
  <p>I get an output of 0 rows. Why are the NaN rows not being recognized as NaN? I have verified that these NaN values are of the type <code>numpy.float64</code>, so they aren't strings that need to be converted. Why are they not recognized as NaNs sometimes?</p>