Problem adding grouped products to the cart

[pkarw]

https://www.dropbox.com/s/o85ma2e8735rlam/Screenshot%202018-04-04%2012.50.56.png?dl=0

It seems like problem with searching by SKU, preparing the grouped product

[pkarw]

By some reason this simple sub-product has type_id = 'simple' and 'configurable_children' set; seems like some residual data

https://www.dropbox.com/s/z6iezp9jq0lunya/Screenshot%202018-04-04%2013.10.35.png?dl=0

[pkarw]

We need to add type_id==='configurable' check to https://github.com/DivanteLtd/vue-storefront/blob/9e40c1afb338c9bf6e147a60461c85fd33ae14e6/core/helpers/index.js#L30 as well