title: Merge with a pivot table changes the column names
body: <p>I am trying to merge a pandas dataframe with a pivot table and it changes the column names. Can I retain the original column names from pivot without having them merged into a single column?</p>
  
  <pre><code>df1:
  
    pn    number_of_records   date    
  A103310     0               2017-09-01  
  B103309     0               2017-06-01
  C103308     0               2017-03-01
  D103307     2               2016-12-01
  E103306     2               2016-09-01
  </code></pre>
  
  <p>df2 which is a pivot table:</p>
  
  <pre><code>  pn                   t1
                    a1    b1    c1
  A103310         3432    324   124
  B103309         342     123   984
  C103308         435     487   245
  D103307         879     358   234
  E103306         988     432   235
  </code></pre>
  
  <p>doing a merge on this dataframe gives me:</p>
  
  <pre><code>df1_df2 = pd.merge(df1,df2,how="left",on="pn")
  </code></pre>
  
  <p>gives me the column names as:</p>
  
  <pre><code>pn  number_of_records date (t1,a1)  (t1,b1)  (t1,c1)
  </code></pre>
  
  <p>How can I instead have them as:</p>
  
  <pre><code>pn  number_of_records date         t1
                               a1    b1   c1
  </code></pre>
  
  <p>in the dataframe after the merge?</p>