title: Entity Framework null should invoke SQL default on insert/update
body: <p>Using EF6, during insert/update, I'd like to be able to invoke the DB default logic on null but override that default if I pass in a value.</p>
  
  <p>Currently it seems I only have two mutually exclusive options:</p>
  
  <ol>
  <li><p>Out of the box a null passed in is written to the record</p></li>
  <li><p>If I change the entity field property to "Computed" on the EF model it uses the DB default logic to populate (eg getdate()) - but ignores what I pass in</p></li>
  </ol>
  
  <p>This is a DB first project using .NET Framework 4.6</p>
  
  <p>Use cases:
  Created / Modified date time columns in the DB defaulting to getdate()
  Defaulting other non-null fields</p>
  
  <p>There are old posts that this can't be done (<a href="https://stackoverflow.com/questions/15531240/entity-framework-database-defaults-on-inserts-allow-updates">4 years ago</a>) and then a post from this summer that it had been resolved in .NET Core, but no info on the details and if the same resolution is also available in .NET Framework.</p>