Core/Spell: Spell ranking downscaling does not work properly

Description:

Level 80 Warrior is able to apply Battle Shout Rank 9 to Level 39, when in fact it should downscale to Rank 3 (just crude example)

Current behaviour:

Apply any buff in theory to lower level and they should receive full rank buff.

Expected behaviour:

Applied buff SHOULD downscale to whatever rank that player is able to receive.

Steps to reproduce the problem:

1. Create 80 char (Example, warrior), train it fully.
2. Create level 39 character (any).
3. Apply Battle shout on lower level target.
4. Lower level should receive 550 Attack power, which is wrong.

Branch(es):

3.3.5, master is not affected as it does not count based on flat value, rather cata+ rely on percentage value.

TC rev. hash/commit:

TrinityCore rev. 0b0bbe4 2018-09-25

Operating system: Unix Debian 9 x64