Updating a cart item via REST API with quantity 0 should remove the cart item

Updating a cart item via REST API with quantity 0 should remove the item from the quote. But instead it is always set to 1?

### Preconditions

1. Magento 2.2 CE

### Steps to reproduce

1. Create quote for guess user via REST API
2. Add product with quantity > 1
3. Update cart item with `PUT /rest/V1/guest-carts/CART_ID/items/ITEM_ID` and use `"qty": 0` as part of body.

### Expected result

1. Cart item should be removed from quote

### Actual result

1. Cart item quantity is always set o 1, independently for the value before

**Original Report**: https://github.com/magento/magento2/issues/16214 by @mhaack

Start by reproducing the guest-cart request at PUT /rest/V1/guest-carts/CART_ID/items/ITEM_ID with an item quantity of 0 in Magento 2.2. Verify that the current behavior changes the quantity to 1; the fix is complete when the cart item is removed from the quote instead.

Written by the indexing model from the issue text.