Request params with nil values are stripped from the notification

Hi,

We've been trying to debug a couple of confusing exceptions this morning, and it turned out part of the confusion was due to Bugsnag removing any request params with nil values from the notification before sending. This is due to `Bugsnag::Helpers.cleanup_obj` being called on the metadata hash.

As nil params can still have an effect (passing nil to a Rails nested attributes setter was the specific problem we had), stripping these params out completely makes debugging a bit tricky...

Cheers,
Simon