title: Pandas fillna on a multiindex series
body: <p>The <code>series_A</code> with NAs is indexed by a MultiIndex <code>(X, Y)</code>, while the values to fill in are in the <code>Series_B</code>, which is indexed by <code>X</code>. How to effectively approach this kind of problem?</p>
  
  <p>For example, here is the <code>series_A</code>:</p>
  
  <pre><code>bar  one    0.299368
       two         NaN
  baz  one   -0.863838
       two   -0.251905
  foo  one    1.063327
       two         NaN
  qux  one    0.206053
       two    0.408204
  </code></pre>
  
  <p>and <code>series_B</code> contains values to fill in:</p>
  
  <pre><code>bar  0.123
  foo  0.456
  </code></pre>