Use correct scheme strings in `bbp_verify_nonce_request()`. Fixes false negative bbPress-specific nonce checks in functions and methods that do not use WordPress's traditional nonce/die pairing. Fixes #2492. Props MZAWeb. (trunk)

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5239 9866e705-20ec-0310-96e7-cbb4277adcfb