Don't do a strict type check when deciding on bp_activity_add_user_favorite() return value

Since r8132, bp_activity_update_meta() has returned an integer value when
falling through to add_metadata(). Thus, we should have a more generous test
for whether the update has been successful. See #5399

Fixes #5515

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8237 cdf35c40-ae34-48e0-9cc9-0c9da1808c22