Core/GameObject: Dismount the player when using spellcaster gameobject

Closes issue #2539.

--HG--
branch : trunk

src/server/game/Entities/GameObject/GameObject.cpp

@@ -1403,6 +1403,7 @@ void GameObject::Use(Unit* user)
                     return;
             }
 
+            user->RemoveAurasByType(SPELL_AURA_MOUNTED);
             spellId = info->spellcaster.spellId;
 
             AddUse();