title: Fill NA values by a two levels indexed Series
body: <p>I have a dataframe with columns (<code>A</code>, <code>B</code> and <code>value</code>) where there are missing values in the <code>value</code> column. And there is a Series indexed by two columns (<code>A</code> and <code>B</code>) from the dataframe. How can I fill the missing values in the dataframe with corresponding values in the series?</p>