Read Repair

[mattsnyder]

Would be nice to perform read repair and remove dead links from the robject from within Ripple as objects are retrieved from Riak.

Currently operations like #count on a Ripple::Document use keys.count which includes dead links.
https://github.com/basho/ripple/blob/master/lib/ripple/associations/many_linked_proxy.rb#L11-14

Iterating over collections also requires rejecting nil values that result from dead links.

My thought is to perform link cleanup after calls to find_target.
https://github.com/basho/ripple/blob/master/lib/ripple/associations/many_linked_proxy.rb#L48-59

Here are tests to describe the desired behavior that could be added to https://github.com/basho/ripple/blob/master/spec/ripple/associations/many_linked_proxy_spec.rb:

 describe "#instantiate", :focus => true do
    context "all links are dead" do
      it "should remove links that could not be instantiated" do
        @person.robject.links << @task.to_link("tasks")
        @person.tasks.inspect # trigger load_object
        @person.robject.links.should be_empty
      end
    end
    context "only some links are dead" do
      it "should remove links that could not be instantiated" do
        @person.robject.links << @task.to_link("tasks")
        @person.tasks << @other_task
        @person.tasks.inspect
        @person.robject.links.should have(1).item
        @person.robject.links.should include(@other_task.to_link("tasks"))
      end
    end
    context "owner also links to objects of another type" do
      it "should only remove dead links of the accessed association" do
        @person.profile = @profile
        @person.robject.links << @task.to_link("tasks")
        @person.robject.links.should have(2).item
        @person.tasks.inspect
        @person.robject.links.should have(1).item
      end
    end
  end