Repo.update! should not execute an UPDATE when the only changed field is the updated_at timestamp

[ericentin]

If you call Repo.update! with a changeset for a schema with timestamps, the DB is sent an UPDATE even if the only changed field is the updated_at timestamp. After a discussion of the issue with @josevalim in #elixir-lang, the conclusion was that the default behavior should probably be to not produce an UPDATE.