Fix backend Add Product to Cart UI

The request was sent but the view was not being updated properly.

I started to notice it after reading #2666. When adding a product to cart through backend the form and order_summary are not updated properly.

I'm aware that the spec added do not pass so I don't expect it to be merge instantly. Just wanted to suggest a fix. I should take a closer look to fix that spec later today though. I'm not 100% sure but I think that are no integration specs to reproduce the whole checkout workflow through backend.

While working on it I also notice that the search itself is broken. e.g. Try searching for "T-Shirt" on a clean Spree install (current master edge at 11c9ce913531d19fd4ab19d7f95c8475c1a63cb1). It just returns all products, no filter at all. Or am I missing something? Hopefully I can take a look at this later too.

cheers