Use plural 'type' in front matter defaults for pages/posts/drafts

Fixes #2657
Ask about 'Draft' before asking about 'Post'

`Draft`s are a subclass of `Post` so `draft.is_a?(Post)` will return
`true`, thus making all `Draft`s `Post`s, which is not desired. If
asking about `Draft` first, then we avoid this problem.

Fixes #2726
FrontmatterDefaults#update_deprecated_types deals with a set, not a scope
Forgot a question mark == broken tests.
Ensure the new type works.
Merge pull request #2732 from jekyll/type-should-be-similar-in-number