The taint flag is not propagated at JSON.parse

I confirmed that the taint flag is not propagated in `JSON.parse`,`JSON.parse!`, `JSON.load`.

$ irb
irb(main):001:0> require 'json'
=> true

# dump
irb(main):002:0> hash = {"key".taint => "value".taint}.taint
=> {"key"=>"value"}
irb(main):003:0> json_str = JSON.dump(hash)
=> "{\"key\":\"value\"}"
irb(main):004:0> json_str.tainted?
=> false
irb(main):005:0>

# prepare tainted string …