[TW] Core/Spell: Rework SPELL_AURA_OBS_MOD_HEALTH

- Fixes spells where value = 1 eg.(Blood Craze, Glyph of Ghost Wolf)
when affected by auras with SPELL_AURA_MOD_HEALING_DONE_PERCENT
(Arena/Bg Dampening)
Merge branch 'patch6' of https://github.com/gecko32/TrinityCore into patch6