[7.x] Fix issue where HasMany relationship wasn't being resolved

[duncanmcclean]

This pull request fixes an issue where HasMany relationships weren't being resolved if the field handle differed from the relationship_name.

For example:

-
  handle: shipping_addresses
  field:
    resource: organization_shipping_address
    relationship_name: shippingAddresses
    type: has_many
    display: 'Shipping addresses'
    sortable: false

Even though the field handle was the "same" as the relationship name, but slugified, doing $model->shipping_addresses wouldn't work.

This PR fixes it by getting the correct relationship name, and calling that attribute instead.

Fixes #624.

[github-actions]

Released as part of v7.12.0.