Fix PrepareDefault#callable! default check with false

[nbcraft]

This PR fixes the callable! check which was not raising when setting default: false, even though it does when setting default: true.

⚠️ This is a breaking change! As it will break for everyone who has set default: false in the past.

• I believe this should trigger a major version bump because of it.
• Not sure if I need to update changelog and version number myself, lmk and I'll update this PR.

(Note: I re-used some of @skryukov code from #93 for specs shared_examples)