implement withFilter for scala 2.10.* for comps

This fixes a compiler warning in scala 2.10 I [commented on](https://github.com/json4s/json4s/issues/29#issuecomment-17614368) a while back. I've been using a [work around](https://gist.github.com/softprops/5934750) at work and wanted to push this up stream. Funny enough it looks like you started down this path but stopped at some point. I came up with something similar and have been using it in production for a while now so you can feel safe to merge. Let' me know otherwise.