Squashed 'json/' changes from 6b3cac42b..d21ed578a

d21ed578a Merge pull request #628 from json-schema-org/catch-unknown-keywords
42b4c3645 Ensure we don't use unknown keywords unless intended.

git-subtree-dir: json
git-subtree-split: d21ed578aeb13b55dfe3604db53adb4750e1abc3

Author: Julian