title: Pandas merge columns, but not the 'key' column
body: <p>This may seem like a stupid question, but this has been bugging me for some time.</p>
  
  <p>df1:</p>
  
  <pre><code>imp_type    value
  1           abc
  2           def
  3           ghi
  </code></pre>
  
  <p>df2:</p>
  
  <pre><code>id          value2
  1           123
  2           345
  3           567
  </code></pre>
  
  <p>Merginge the 2 df's:</p>
  
  <pre><code>df1.merge(df2, left_on='imp_type',right_on='id')
  </code></pre>
  
  <p>yields:</p>
  
  <pre><code>imp_type    value    id    value2
  1           abc      1     123
  2           def      2     345
  3           ghi      3     567
  </code></pre>
  
  <p>Then I need to drop the <code>id</code> column since it's essentially a duplicate of the imp_type column. Why does merge pull in the join key between the 2 dataframes by default? I would think there should at least be a param to set to False if you don't want to pull in the join key. Is there something like this already or something I'm doing wrong?</p>