FIX(add cart item): add item in cart by increment and decrement

[Hakizimana-Clement]

What does this PR do?

This PR is fix on cart endpoint where on create cart, the buyer only decrement quantity in cart without without decrement.

Description of Task to be completed?

1. when buyer add product in cart, can increment/decrement quantity of product in cart.

How should this be manually tested?

1. Clone repository
2. Switch to branch fix-cart-187419072
3. install packages and have all envs as they appear in .env-examples
4. Try to add product in cart with productId and quantity and then change product quantity of the same product and see how it increment and decrement. according to the quantity you add.

[codeclimate]

Code Climate has analyzed commit 1de4126 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (60% is the threshold).

This pull request will bring the total coverage in the repository to 97.1% (0.0% change).

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