Fix warning from trying to access an uninitialized ivar.

Signed-off-by: Ryan Davis <zenspider@chef.io>

lib/json/pure/generator.rb

@@ -250,7 +250,8 @@ def [](name)
           if respond_to?(name)
             __send__(name)
           else
-            instance_variable_get("@#{name}")
+            instance_variable_get("@#{name}") if
+              instance_variables.include?("@#{name}".to_sym) # avoid warning
           end
         end