title: Merging dataframes with unhashable columns
body: <p>I want to merge two Pandas DataFrames. If the <code>item</code> code (e.g. A,B,C,D) are the same,
  their attributes <code>a</code>, <code>b</code> must be the same, but <code>b</code> is a numpy array or a list that is
  unhashable.</p>
  
  <p>Foo:</p>
  
  <pre><code>item   a     b              
  A      1     [2,0] 
  B      1     [3,0]         
  C      0     [4,0]         
  </code></pre>
  
  <p>Bar:</p>
  
  <pre><code>item   a     b
  A      1     [2,0]
  D      0     [6,1]
  </code></pre>
  
  <p>This is what I want</p>
  
  <pre><code>code   a     b        Foo   Bar
  A      1     [2,0]    1     1
  B      1     [3,0]    1     0
  C      0     [4,0]    1     0
  D      0     [6,1]    0     1
  </code></pre>