Core/Entities: Units with spirit of redemption can not attack (fixes #8623)

Reproduction steps:

1) You need two characters of opposing factions. One of them priest with talent Guardian Spirit (Holy).

2) Make sure, that players can attack each other (activate PvP)

3) Get players in melee range of each other

4) On priest, create and execute macro:
/tar PRIESTNAME
.ca 5
/tar OTHERPLAYERNAME
/startattack

5) Other player can attack priest now and gets one honorable kill for every hit.


RemoveAurasByType(SPELL_AURA_MOD_UNATTACKABLE) in bool Unit::Attack(Unit* victim, bool meleeAttack) causes the unattackable aura to be removed instantly:

>	worldserver.exe!Unit::RemoveAurasByType(AuraType auraType, unsigned __int64 casterGUID, Aura * except, bool negative, bool positive) Line 3881	C++
 	worldserver.exe!Unit::Attack(Unit * victim, bool meleeAttack) Line 9093	C++
 	worldserver.exe!WorldSession::HandleAttackSwingOpcode(WorldPacket & recvData) Line 69	C++
 	worldserver.exe!WorldSession::Update(unsigned int diff, PacketFilter & updater) Line 320	C++
 	worldserver.exe!World::UpdateSessions(unsigned int diff) Line 2860	C++
 	worldserver.exe!World::Update(unsigned int diff) Line 2196	C++
 	worldserver.exe!WorldRunnable::run() Line 61	C++
 	worldserver.exe!ACE_Based::Thread::ThreadTask(void * param) Line 186	C++


Description and more information in bugreport
#8623

Environment:
Tested it under Win with:
TC: 895a23d
DB: TDB 52