title: Set value using loc on part of multiindex
body: <p>I set multiindex on dataframe</p>
  
  <pre><code>df.set_index(['A', 'B', 'C'])
  </code></pre>
  
  <p>then I want to set some value to all rows, where index 'A' equals 'foo'</p>
  
  <pre><code>df.loc[df['A'] == 'foo'] = 5
  </code></pre>
  
  <p>it raises following error:</p>
  
  <pre><code>KeyError: 'A'
  </code></pre>