Rails 4.0.1以降へのアップデート後に`ARModel.create`が`NoMethodError: undefined method '[]' for nil:NilClass`というエラーを吐く

## 現象
Rails 4.0.1へアップデート後に`some_ar_base_class.create`を呼ぶと以下のようなエラーが出るようになった．

```
[1] pry(main)> User.create
NoMethodError: undefined method `[]' for nil:NilClass
from /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/attribute_methods/read.rb:86:in `block in read_attribute'
```

スタックトレースを見ても謎

```
     NoMethodError:
       undefined method `[]' for nil:NilClass
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/attribute_methods/read.rb:86:in `block in read_attribute'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/attribute_methods/read.rb:84:in `fetch'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/attribute_methods/read.rb:84:in `read_attribute'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/attribute_methods/time_zone_conversion.rb:38:in `notification_last_seen='
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/attribute_assigner.rb:16:in `block (2 levels) in object'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/attribute_assigner.rb:15:in `each'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/attribute_assigner.rb:15:in `block in object'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/attribute_assigner.rb:14:in `tap'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/attribute_assigner.rb:14:in `object'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/evaluation.rb:12:in `object'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/strategy/create.rb:9:in `result'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/factory.rb:42:in `run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/factory_runner.rb:23:in `block in run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activesupport-4.0.2/lib/active_support/notifications.rb:161:in `instrument'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/factory_runner.rb:22:in `run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/factory_girl-4.3.0/lib/factory_girl/strategy_syntax_method_registrar.rb:20:in `block in define_singular_strategy_method'
     # /Users/yaotti/src/qiita/spec/spec_helper.rb:99:in `foo'
     # /Users/yaotti/src/qiita/spec/spec_helper.rb:62:in `block (2 levels) in <top (required)>'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/hooks.rb:21:in `instance_eval'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/hooks.rb:21:in `run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/hooks.rb:100:in `run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/hooks.rb:418:in `run_hook'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:320:in `block in run_before_all_hooks'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/memoized_helpers.rb:96:in `block in isolate_for_before_all'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/memoized_helpers.rb:92:in `instance_eval'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/memoized_helpers.rb:92:in `isolate_for_before_all'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:319:in `run_before_all_hooks'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:370:in `run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `block (2 levels) in run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `map'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `block in run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/reporter.rb:34:in `report'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:25:in `run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:80:in `run'
     # /opt/boxen/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:17:in `block in autorun'
```

## 解決策
`protected_attributes` gemを1.0.5以降にアップデートする．
関連しそうなコミット: [Override the proper hook to make assign_attributes work with options · 22e1953 · rails/protected_attributes](https://github.com/rails/protected_attributes/commit/22e1953ae68850bafbb0c5939b45922d89907d67)
Rails 3から4に移行した人は`protected_attributes`を使っていることが多そうなのでメモ．