move actual activation to model

fixes #1881

test/models/todo_test.rb

@@ -542,4 +542,14 @@ class TodoTest < ActiveSupport::TestCase
     assert_equal "<p><strong>test</strong></p>", todo.rendered_notes
   end
 
+  def test_destroying_action_activates_successors
+    @not_completed1.add_predecessor(@not_completed2)
+    @not_completed1.block!
+
+    @not_completed2.destroy
+
+    @not_completed1.reload
+    assert @not_completed1.active?
+  end
+
 end