Don't concat extnames in output for multiple converters
Specify Ruby Dev Headers on Installation docs page.

Closes #2728
Call #to_liquid before calling #to_json in jsonify filter.
Merge pull request #2729 from jekyll/to_liquid_before_jsonify
Update history to reflect merge of #2729 [ci skip]
Merge pull request #2722 from jekyll/don-t-concat-extnames
Update history to reflect merge of #2722 [ci skip]
Document#to_s should produce just the content of the document.

Fixes #2655
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
Update history to reflect merge of #2732 [ci skip]
Just like for Convertible, place the un-layouted content in document.content.
Merge pull request #2731 from jekyll/doc-to_s
Update history to reflect merge of #2731 [ci skip]
Update history to reflect merge of #2731 [ci skip]