Add merge method to Base OrderItemInterface

Signed-off-by: Benoît Burnichon <bburnichon@gmail.com>
Add Base OrderItem specs for merge implementation

Signed-off-by: Benoît Burnichon <bburnichon@gmail.com>
Add Order specs to use merge method of its items

Signed-off-by: Benoît Burnichon <bburnichon@gmail.com>