When having two pieces of Armor with (negative) MaxHealth attribute, equiping one and then taking the other one into your Main-/Offhand slot and switching your Main-/Offhand item to something else makes the game ignore the (negative) MaxHealth attribute of the piece of armor your wearing.
(Look at the healthbar in the screenshots)

Command for recreation:

/give @p minecraft:diamond_helmet 1 0 {AttributeModifiers:[

{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:-0.9,Operation:1,UUIDLeast:408588,UUIDMost:100271}

]}