servlet attributes with null / false values

[due](https://github.com/jruby/jruby-rack/blob/master/src/main/ruby/rack/handler/servlet/default_env.rb#L116) discovered while :scissors: at https://github.com/killbill/killbill-plugin-framework-ruby/commit/2db1514adb2025ff3694ec181a03f9b5e54b3363#commitcomment-10770544
