Querying notifications by string format fails

[r-a-y]

See:
​https://buddypress.org/support/topic/buddypress-2-6-0-espejo-is-now-available/#post-255213

Problem was introduced by me! Forgot to pass the $format variable as 'string'.

[r-a-y]

In 10910:

Notifications: Fix issue with querying notifications by string format.

If someone is using:
bp_notifications_get_notifications_for_user( $user_id, 'string' )

This functionality broke in #7066.

Commit fixes this issue and includes unit tests.

Fixes #7141 (2.6-branch).

[r-a-y]

In 10911:

Notifications: Fix issue with querying notifications by string format.

If someone is using:
bp_notifications_get_notifications_for_user( $user_id, 'string' )

This functionality broke in #7066.

Commit fixes this issue and includes unit tests.

Fixes #7141 (trunk).