Use AR#attributes instead of the instance variable

As of Rails 3.2.12, they can return different things.
For example, if the `foorbars` table has an integer column called "size" with default 0 and not null:
```
f = Foobar.new
f.attributes["size"].class #=> Fixnum
f.instance_eval{@attributes}["size"].class #=> Arel::Nodes::SqlLiteral
```
This fixes an issue where:
```
Foobar.new.to_yaml
```
would raise an error:
```
ArgumentError: wrong number of arguments (2 for 1)
	from gems/arel-3.0.2/lib/arel/expressions.rb:3:in `count'
```