Sanitize extra data passed into sentry

The need for this arose through use of the celery integration, which [passes task arguments as extra data](https://github.com/getsentry/raven-python/blob/5.1.1/raven/contrib/celery/__init__.py#L38). The [SanitizePasswordsProcessor docstring](https://github.com/getsentry/raven-python/blob/5.1.1/raven/processors.py#L64]) implies this is expected behavior.
Shouldn't it be data['extra'] here ?