Fix Marshal serialization of stubbed object

[txus]

Inspired in the solution for yaml serialization :)
It is a bit monkey-patchesque but I think it is justified, because the Marshal extension is only loaded when Serialization module is.

Patched in
http://github.com/txus/rspec-mocks/commit/b003364efd3a6fbac6b431ccb3b78f5d0d8641d4

[txus]

Fix Marshal serialization of stubbed objects

• Closed by 80ee73d.