title: Pandas fillna on datetime object
body: <p>I'm trying to run <code>fillna</code> on a column of type datetime64[ns]. When I run something like:
  <code>df['date'].fillna(datetime("2000-01-01"))</code></p>
  
  <p>I get: 
  <code>TypeError: an integer is required</code></p>
  
  <p>Any way around this?</p>