Indifferent access for model's error messages

I think `full_messages_for` should be consistent with `errors`. Before it was like this 

```ruby
class Post
  include ActiveModel::Validatations
  attr_accessor :title
  validates_presence_of :title
end

post = Post.new
post.valid?

post.errors[:title] # => 'can't be blank'
post.errors['title'] # => 'can't be blank'

post.errors.full_messages_for :title # => ['Title can't be blank']
post.errors.full_messages_for 'title' # => []
```