title: Rails Upsert PG created_at issue
body: <p>Sorry if this has been answered before but I can't find a definite answer.</p>
  <p>Surely someone has come across this issue when using <code>upsert</code> you need to specify every column including <code>created_at</code> and <code>updated_at</code>?</p>
  <p>How can I not update the <code>created_at</code> timestamp when an upsert happens? I only want to insert <code>created_at</code> once.</p>
  <pre><code>a = User.upsert({
            name: ....,
            age: ....
            created_at: DateTime.now, 
            updated_at: DateTime.now
          }, unique_by: :upsert_user_index )
  </code></pre>