Workaround for JRuby

Author: hsbt

lib/json/common.rb

@@ -71,6 +71,7 @@ def generator=(generator) # :nodoc:
       end
       self.state = generator::State
       const_set :State, self.state
+      const_set :SAFE_STATE_PROTOTYPE, State.new # for JRuby
     ensure
       $VERBOSE = old
     end