[#3073] Define filter() in Decompress filter

- and have it mimic __invoke()
[#3073] Remove namespace from interface reference
Merge pull request #3487 branch 'hotfix/allow-traversable-in-captcha'
Merge commit 'refs/pull/3486/head' of github.com:zendframework/zf2 into hotfix/decompress-filter
Forward port #3486