Added default support to DummyValueCreationSession for type Lazy<T>

Based off of the behavior that is defined for Task. Discussed in issue 358 https://github.com/FakeItEasy/FakeItEasy/issues/358
I appreciate the addition of the Specs, but am not entirely sure of the usefulness of checking that the Lazy returns the same value twice—that seems more about testing the functionality of Lazy itself. If it makes sense to you, I'd consider instead testing that

1. We get a non-null Lazy back (when the type argument is Dummyable)
1. The Lazy's `IsValueCreated` property is initially `false` (maybe this is not strictly necessary)
1. Accessing the `Value` is, in fact, a Dummy (one way to do this is to set up a new DummyDefinition like the `GuidDefinition` one in the integration tests) (maybe I'm getting too paranoid here)

Feel free to rebut, of course.