Pass 'string' rather than 'simple' as default parameters in `bp_core_get_notifications_for_user()` and `bp_notifications_get_notifications_for_user()`. Fixes #BB2508. (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7732 cdf35c40-ae34-48e0-9cc9-0c9da1808c22