* ext/json, test/json: merge JSON HEAD(17fe8e7)
  https://github.com/flori/json/compare/v1.8.1...17fe8e7

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49051 b2dd03c8-39d4-4d8f-98ff-823fe69b080e