Bugfix for calculate_attack()
Bugfix for calculate_attack()

The attack_delta can be negative, making the attack below 0. If below 0,
set it to 0.

noa/hearthbreaker,slaymaker1907/hearthbreaker,Ragowit/hearthbreaker,kingoflolz/hearthbreaker,jirenz/CS229_Project,slaymaker1907/hearthbreaker,danielyule/hearthbreaker