Friendship model: Dependent destroy doesn't work correctly on friend association

[lucasbiguet]

When a friend or friendable is destroyed it should destroy all its friendships too. It works out of the box on the friendable relation but not on the friend one

Here is a set of specs illustrating the problem:

context "Callbacks" do
  let(:friend) { create :user }
  let(:friendable) { create :user }
  let!(:friendship) { HasFriendship::Friendship.create(friend: friend, friendable: friendable, status: "accepted") }

  it "destroys friendship on friend destroy" do
    # Spec doesn't pass :(
    expect { friend.destroy }.to change { HasFriendship::Friendship.count }.by(-1)
  end

  it "destroys friendship on friendable destroy" do
    # Spec succeeds
    expect { friendable.destroy }.to change { HasFriendship::Friendship.count }.by(-1)
  end
end

I will try to submit a PR if I can find time to work on this