Remove dangerous query method warning

[potomak]

Warning message:

DEPRECATION WARNING: Dangerous query method (method whose arguments
are used as raw SQL) called with non-attribute argument(s):
"coalesce(spree_shipments.shipped_at, spree_shipments.created_at)
desc". Non-attribute arguments will be disallowed in Rails 6.0.
This method should not be called with user-provided values, such as
request parameters or model attributes. Known-safe values can be
passed by wrapping them in Arel.sql(). (called from block in
<class:Shipment> at solidus/core/app/models/spree/shipment.rb:37)

SQL generated by the scope before this update:

> Spree::Shipment.reverse_chronological.to_sql
 => "SELECT \"spree_shipments\".* FROM \"spree_shipments\" ORDER BY
  coalesce(spree_shipments.shipped_at, spree_shipments.created_at)
  desc, \"spree_shipments\".\"id\" DESC"

SQL generated by the scope after this update:

> Spree::Shipment.reverse_chronological.to_sql
 => "SELECT \"spree_shipments\".* FROM \"spree_shipments\" ORDER BY
  COALESCE(\"spree_shipments\".\"shipped_at\",
  \"spree_shipments\".\"created_at\") DESC,
  \"spree_shipments\".\"id\" DESC"

[potomak]

The same issue has been addressed by #2921. The approach is slightly different, but both PRs fix this deprecation warning.

[kennyadsl]

I'm sure this is a great way to fix the issue but I'm not an Arel expert so I'm more for the easier to understand solution provided in #2921 . But let's wait to see if we have other opinions here.

In the meantime thanks for the contribution!