acts_as_tree patch

Acts as Tree users please check out ticket #10843

Please submit a diff rather than the whole file, as well as some tests proving that your code works. Please follow the instructions on

My bad.

I've attached a .diff with additions to the test suite.

I'm concerned about defaulting to :dependent => :destroy. Seems like
a breaking change. Existing code that relied on the current default
behavior might not work as expected anymore.