Previously, freezing a cloned ActiveRecord object froze the original
too. By cloning `@attributes` before freezing, we prevent cloned objects
(which in Ruby share state of ivars) from being effected by `#freeze`.
Resolves issue #4936, which has further information on this issue, as
well as steps to reproduce.
* Add a test case for `#freeze` not causing `cloned.frozen?` to be true.
* Clone @attributes before freezing in `ActiveRecord::Core`, then
reassign the cloned, frozen hash to the frozen model's `@attributes`
ivar.
/cc @steveklabnik