title: pandas fillna datetime column with timezone now
body: <p>I have a pandas datetime column with <code>None</code> values which I would like to fill with <code>datetime.now()</code> in a specific timezone.</p>
  
  <p>This is my MWE dataframe:</p>
  
  <pre><code>df = pd.DataFrame([
      {'end': "2017-07-01 12:00:00"},
      {'end': "2017-07-02 18:13:00"},
      {'end': None},
      {'end': "2017-07-04 10:45:00"}
  ])
  </code></pre>
  
  <p>If I fill with <code>fillna</code>:</p>
  
  <pre><code>pd.to_datetime(df['end']).fillna(datetime.now())
  </code></pre>
  
  <p>The result is a series with expected dtype: <code>datetime64[ns]</code>. But when I specify the timezone, for example:</p>
  
  <pre><code>pd.to_datetime(df['end']).fillna(
      datetime.now(pytz.timezone('US/Pacific')))
  </code></pre>
  
  <p>This returns a series with dtype: <code>object</code></p>