json/add/range.rb: use Range#end

* ext/json/lib/json/add/range.rb (Range#as_json): use Range#end
  instead of Range#last which can be affected by Range#exclusive?.
  ref [Bug #8739]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42404 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/json/lib/json/add/range.rb

@@ -16,7 +16,7 @@ class Range
   def as_json(*)
     {
       JSON.create_id  => self.class.name,
-      'a'             => [ first, last, exclude_end? ]
+      'a'             => [ first, self.end, exclude_end? ]
     }
   end