Add merge method to Base OrderItemInterface

Signed-off-by: Benoît Burnichon <bburnichon@gmail.com>
Make Core OrderItem equals method to check other item type

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>
Merge pull request #809 from bburnichon/master

Order addItem method should merge two equal OrderItems