Instantiate through initialize_copy when cloning

As discussed here I'm attaching a patch which overrides #initialize_copy rather than #clone in order to implement cloning of AR objects. This will allow AR classes to implement deep cloning of attributes much more easily, for example:

  def initialize_copy(other)
    super
    self.an_association = other.an_association
  end

This patch isn't intended to be the finished article as I have two particular points on which I'd appreciate some feedback.

Firstly, I'm having to call the after_initialize callback near the top of #initialize_copy in order to get the test suite to pass. This doesn't seem right to me and I've documented my thoughts in the patch file.

Secondly, #initialize allows a block to be passed in. Should #initialize_copy do a similar thing? If so I'll add a test and relevant code.