Failing test for null params stripping

Bugsnag::Helpers.cleanup_obj removes any keys from a hash whose value is
nil. As it is called on the notification metadata before sending, this
removes any nil request params, e.g. a JSON POST of:

    { "nil_param": null }

is reported as having an empty params hash.
Merge pull request #37 from urbanautomaton/null-request-params-stripped

Request params with nil values are stripped from the notification