Fix on_(grant|revoke) not being run by mods

[appgurueu]

Trivial, untested fix for mods setting privs not running on_grant / on_revoke anymore.

(Issue originally reported by FatalError on Discord)

[rubenwardy]

Make sure to test for mutation (get_auth_entry does return a new table so pretty sure this will work)

local privs = minetest.get_player_privileges(name)

privs.fly = true
minetest.set_player_privileges(name, privs)

privs.fast = true
minetest.set_player_privileges(name, privs)

[appgurueu]

Ok, tested (first defined a test priv which prints granted / revoked, then set the privs to just test, then to the empty set), works.