[#558] Cast collection properties as empty Array by default

Previously, omitting value for casted collection resulted in the property being `nil`.

Now, with a class definition like this:

    class Article
      include Tire::Model::Persistence

      # ...
      property :comments, :class => [Comment]
    end

the `comments` property is by default casted as an empty Array:

    article = Article.new :title => 'Test'
    article.comments
    # => []

Closes #558