title: Pandas series Timestamp converted to numpy.datetime64 after applying unique()
body: <p>I have a <code>pd.Series</code> whose elements are <code>pd.Timestamp</code>. However, after applying <code>unique()</code> to the series, the type has been converted to <code>numpy.datetime64</code> and is represented in a different time zone. This behavior is a bit inconvenient because typically <code>unique()</code> isn't expected to change the type. Is this behavior expected?</p>
  
  <pre><code>In [49]: dates
  Out[49]: 
  
       ym
  0    2013-11-01
  1    2013-12-01
  2    2014-01-01
  3    2014-02-01
  4    2014-03-01
  
  In [50]: dates.ym[0]
  Out[50]: Timestamp('2013-11-01 00:00:00')
  
  In [51]: dates.ym.unique()[0]
  Out[51]: numpy.datetime64('2013-10-31T17:00:00.000000000-0700')
  </code></pre>