Update to work with latest version of the gem

[alexspeller]

Some minor changes to how associations are tested, and allow specification of attribute aliases, e.g.

  it "should match attributes with keys" do
    serializer = Class.new ActiveModel::Serializer do
      attribute :foo, :key => :foo_name
    end

    serializer.should_not have_attribute :bar
    serializer.should have_attribute :foo
    serializer.should have_attribute(:foo).as(:foo_name)
    serializer.should_not have_attribute(:foo).as(:another_name)
  end