title: insert_all does not create auto generated inserted_at with ecto 2.0
body: <pre><code># mix.exs
    defp deps do
      [{:phoenix, "~&gt; 1.1.4"},
        {:postgrex, "&gt;= 0.0.0"},
        {:phoenix_ecto, "~&gt; 3.0.0-rc"},
        {:gettext, "~&gt; 0.9"},
        ...
      ]
    end
  </code></pre>
  
  <p>The installed ecto version is <code>"2.0.0-rc.5"</code></p>
  
  <pre><code>iex(1)&gt; categories = [%{name: "stackoverflow", url: "stackoverflow.com"}]
  
  iex(2)&gt; App.Repo.insert_all App.Category, categories
  
  ** (Postgrex.Error) ERROR (not_null_violation): null value in column "inserted_at" violates not-null constraint
  
      table: categories
      column: inserted_at
  
  Failing row contains (1, stackoverflow, stackoverflow.com, null, null).
      (ecto) lib/ecto/adapters/sql.ex:176: Ecto.Adapters.SQL.query!/5
      (ecto) lib/ecto/adapters/sql.ex:350: Ecto.Adapters.SQL.insert_all/8
      (ecto) lib/ecto/repo/schema.ex:42: Ecto.Repo.Schema.do_insert_all/6
  iex(2)&gt; 
  </code></pre>
  
  <p>From the docs, it seemed the <code>inserted_at</code> was autogenerated. Should I've to manually do this part?</p>
  
  <pre><code>schema "categories" do
   field :name, :string
   field :url, :string
  
   timestamps
  end
  </code></pre>