Allow fallback to LegacyKeyGenerator when secret_key_base is not set

Let devs upgrading from older Rails take advantage of the LegacyKeyGenerator without needing a monkeypatch to use a blank secret_key_base

This behavior may have been removed by accident.  See https://github.com/rails/rails/commit/484ff7d29e7a27216219cf5460bf675c38abf522


e.g.

```ruby
Rails::Application.class_eval do
  # the key_generator will then use ActiveSupport::LegacyKeyGenerator.new(config.secret_token)
  fail "I'm sorry, Dave, there's no :validate_secret_key_config!" unless instance_method(:validate_secret_key_config!)
  def validate_secret_key_config! #:nodoc:
    config.secret_token = secrets.secret_token
    if config.secret_token.blank?
      raise "Missing `secret_token` for '#{Rails.env}' environment, set this value in `config/secrets.yml`"
    end 
  end 
end
```

## Update 2014-10-21:

See [some possible future work for Rails exposed in this PR](https://github.com/rails/rails/pull/16535#issuecomment-59997273) such as clarifying what goes in secrets vs. config, and if the two should be synced.  
oops, this is wrong.  it's in the 'Upgrading from Rails 3.2 to Rails 4.0' section.  We might also want to note here to use https://github.com/envato/rails_4_session_flash_backport/ in 3.2 and then in 4.x 

```ruby
# Stay on the :marshal serializer till 4 is stable
# Then we can switch to :hybrid, and eventually to :json
Rails.application.configure do
  config.action_dispatch.cookies_serializer = :marshal
end
```