non-string dict keys in django-celery configs

This fix allows celery-flower to not have problems displaying
configurations for projects that still use configurations embedded
in django settings files.  In this instance there are some int
dict keys that are totally unrelated to celery but that are causing
frequent error messages in the celeryd logs.
Fix handling of non-string keys in filter_hidden_settings
Merge branch '3.1' of github.com:celery/celery into 3.1