Added default support to DummyValueCreationSession for type Lazy<T>

Based off of the behavior that is defined for Task. Discussed in issue #358.
perhaps `should_return_a_lazy_with_a_dummy_value`?

Also, the `.BeAssignableTo` would probably be better off checking that it's assignable to an `IFoo`. That or just check for null, as the original field name said. `ITaggable` doesn't really mean must outside of FakeItEasy itself. (And I'm hoping to remove it from the public API soon anyhow, as part of the investigation in #374.)