Added backword compatible for < json-2.5

Author: hsbt

lib/json/common.rb

@@ -72,6 +72,8 @@ def generator=(generator) # :nodoc:
       self.state = generator::State
       const_set :State, self.state
       const_set :SAFE_STATE_PROTOTYPE, State.new # for JRuby
+      const_set :FAST_STATE_PROTOTYPE, create_fast_state
+      const_set :PRETTY_STATE_PROTOTYPE, create_pretty_state
     ensure
       $VERBOSE = old
     end