title: Pandas dataframe merge skips rows?
body: <p>This seems odd. </p>
  
  <p>There's one data frame (df_ceremonies):</p>
  
  <pre><code>    Year    Ceremony_Date
  0   1927/28 May 16, 1929
  1   1928/29 April 3, 1930
  2   1929/30 November 5, 1930
  3   1930/31 November 10, 1931
  4   1931/32 November 18, 1932
  5   1932/33 March 16, 1934
  6   1934    February 27, 1935
  7   1935    March 5, 1936
  </code></pre>
  
  <p>Here's the second df:</p>
  
  <pre><code>    Year       Ceremony Award   Winner  Name                Film
  0   1927/1928   1       Actor   NaN     Richard Barthelmess The Noose
  1   1927/1928   1       Actor   1.0     Emil Jannings       The Last Command
  2   1927/1928   1       Actress NaN     Louise Dresser      A Ship Comes In
  3   1927/1928   1       Actress 1.0     Janet Gaynor        7th Heaven
  4   1927/1928   1       Actress NaN     Gloria Swanson      Sadie Thompson
  </code></pre>
  
  <p>Here's the command &amp; result.  Notice how the years don't join until 1934, thus skipping 1927/1928, etc. They are all of the object data type. Why so?</p>
  
  <pre><code>df_revised = pd.merge(df, df_ceremonies, on='Year')
  
      Year    Ceremony    Award   Winner  Name            Film          Ceremony_Date
  0   1934    7          Actor    1.0     Clark Gable     It Happened One Night   February 27, 1935
  1   1934    7          Actor    NaN     Frank Morgan    The Affairs of Cellini  February 27, 1935
  2   1934    7          Actor    NaN     William Powell  The Thin Man    February 27, 1935
  3   1934    7         Actress   1.0     Claudette Colbert   It Happened One Night   February 27, 1935
  4   1934    7         Actress   NaN     Bette Davis      Of Human Bondage   February 27, 1935
  </code></pre>