Composite products' associated products do not require an image
Composite products' associated products do not require an image

If a product has no image then `image` attribute usually contains the
value `no_selection` but not always.
Sometimes children are created en masse in a way that avoids triggering
events which would take care of setting `no_selection`.
The `image` attribute was inner joined and that prevented such children
from showing in the `/api/rest/products/:id/associated` resource.
Now it is left joined instead.

Fixes #6

clockworkgeek/Magento-Extra-RESTful