[z2065] Fix sent level in trainer spell list - this fixes problems especially with druid trainers

* This allows to really  overwrite required level for spells
* Adds proper checks when buying
* Fixes default level requirement
Based on commit from mangos-one :
https://github.com/mangos-one/server/commit/0a63b2798cf5c1f3d89564aec4a1
00870e527197

Signed-off-by: Schmoozerd <schmoozerd@scriptdev2.com>

(based on commit 9f52382)

(based on commit be46a2d)