title: pandas groupby - case sensitive issues in groups
body: <p>I need to groupby a DataFrame by a column named "Keyword", with:</p>
  
  <pre><code>grouped = df.groupby('Keyword')
  </code></pre>
  
  <p>Then I'm searching for the size of each group:</p>
  
  <pre><code>a = grouped.size()
  </code></pre>
  
  <p>And the results are like these:</p>
  
  <pre><code>Keyword
  ATTORNEY            48
  Appraiser           94
  Attorney          1437
  BASEBOARD            2
  BELL PEPPER          1
  BULLETIN BOARD       1
  Bell Pepper         36
  Bell pepper         19
  Bulletin Board      20
  Bulletin board       3
  CANDY              765
  CANDy                2
  CANdy                1
  ...                ...
  </code></pre>
  
  <p>I would like to avoid case sensitive issues, and get a unique group for words like "CANDY, CANDy, CANdy" etc, which differ only for some lower or upper characters. I tried to escape this problem setting to <code>df['Keyword'].str.lower()</code> before grouping, but it doesn't work.
  Any help would be really appreciated, thanks.</p>