Allow applying further filters after DoctrineProxyFilter

[fsevestre]

Backport PR #101 and #133 (based issue #98) on version 1, after discussions on #173.

I continue to think that being unable to apply 2 filters with Doctrine Proxy is a bug. If you think it's a BC break, I can create a new DoctrineProxyFilter class and depreciate the existing class, but I'm not sure how to name it.