Register clean up

I would return this explicitly here to be sure that it works fine. I'm not sure the C implementation of Pimple is fluent all the time because of https://github.com/fabpot/Pimple/blob/3764c3985684ad6af4ad3c7d2e0595f9f09faf5f/ext/pimple/pimple.c#L777

The fact that ``Pimple\Container::register`` is fluent is not covered by the testsuite