title: unique of column with unhashable type
body: <p>Two out of three of my columns respond fine to <code>.nunique()</code></p>
  
  <pre><code>print(df.Column1.nunique())
  &gt;&gt;&gt; 45
  print(df.Column2.nunique())
  &gt;&gt;&gt; 23
  </code></pre>
  
  <p>But the other column:</p>
  
  <pre><code>print(df.Column3.nunique())
  </code></pre>
  
  <p>..gives a Traceback, with the following last two lines:</p>
  
  <pre><code>pandas/src/hashtable_class_helper.pxi in pandas.hashtable.PyObjectHashTable.unique (pandas/hashtable.c:14999)()
  
  TypeError: unhashable type: 'RequestsCookieJar'
  </code></pre>
  
  <p>When you look at the df in Jupyter, you can to some extent visually see differences between cookies, one might start like this:</p>
  
  <pre><code>[&lt;Cookie 1P_JAR=2018-03-07-09
  </code></pre>
  
  <p>..the other like this:</p>
  
  <pre><code>[&lt;Cookie 1P_JAR=2018-03-07-08
  </code></pre>
  
  <p>Is there a quick fix so it can tell you how many of them are unique?</p>