Tiny fix for rails4

Consider a serializer class for a field containing an hstore. With ActiveRecord::Coders::Hstore it could look like this:

```ruby
class CustomHstore
  def self.load(value)
    hash = value.from_hstore
    # Do something with hash
    hash
  end

  def self.dump(value)
    ActiveRecord::Coders::Hstore.dump(value)
  end
end
```

Since Rails 4.0 it turns into

```ruby
class CustomHstore
  def self.load(hash)
    # Do something with hash

    hash
  end

  def self.dump(value)
    value
  end
end
```

Based on: http://big-elephants.com/2013-07/rails-4-and-activerecord-4-0/