add attributes instance variable

`@attributes` hash is what activemodel validations looks at to determine successful / unsuccessful validation.

Without this PR, this is what happens:
````
class Foo < CouchRestAdapter::Base
  validates :bar, presence: true
end

f = Foo.new bar: 'value'
f.valid?
> true

f.value = nil

f.valid?
> true
````

This is because `@attributes` is not getting updated. With this PR, it will properly report invalid.