Combat skills should level on low level mobs as well

Guides pre-wotlk seem to all suggest that leveling in TBC daily zones is
possible with a wotlk character (and they are grey to a lvl 80).
http://www.mmo-champion.com/threads/668520-Leveling-weapon-skill

Another source suggests weapon skill levels no matter the target
creature level
https://typhoonandrew.wordpress.com/2008/01/08/how-to-raise-your-weapon-skill/

src/game/Entities/Player.cpp

@@ -5749,13 +5749,11 @@ void Player::UpdateCombatSkills(Unit* pVictim, WeaponAttackType attType, bool de
     uint32 plevel = getLevel();                             // if defense than pVictim == attacker
     uint32 greylevel = MaNGOS::XP::GetGrayLevel(plevel);
     uint32 moblevel = pVictim->GetLevelForTarget(this);
-    if (moblevel < greylevel)
-        return;
 
     if (moblevel > plevel + 5)
         moblevel = plevel + 5;
 
-    uint32 lvldif = moblevel - greylevel;
+    uint32 lvldif = std::abs(int32(moblevel - greylevel));
     if (lvldif < 3)
         lvldif = 3;