Deprecate passing return_items_attributes in API as hash of hashes

This is a behaviour which was previously undocumented worked because of the
native Rails parameter parsing for nested attributes. This is not
something we want to support through the API going forward so we are
adding a deprecation warning for anyone using this behaviour currently.

api/app/controllers/spree/api/customer_returns_controller.rb

@@ -71,6 +71,10 @@ def build_customer_return
 
         if return_items_params.is_a? ActionController::Parameters
           return_items_params = return_items_params.values
+          Spree::Deprecation.warn(
+            "Passing `return_items_attributes` as a hash of hashes is \
+deprecated and will be removed in future versions of Solidus."
+          )
         end
 
         @customer_return = CustomerReturn.new(customer_return_attributes)

api/spec/requests/spree/api/customer_returns_controller_spec.rb

@@ -183,12 +183,22 @@ module Spree
           end
 
           it "can create a new customer return" do
+            expect(Spree::Deprecation).to receive(:warn)
             expect { subject }.to change { Spree::CustomerReturn.count }.
               from(0).to(1)
 
             expect(response).to have_http_status(:success)
             expect(json_response).to have_attributes(attributes)
           end
+
+          it "logs a deprecation warning" do
+            expect(Spree::Deprecation).
+              to receive(:warn).
+              with(
+                /Passing `return_items_attributes` as a hash of hashes is deprecated/
+              )
+            subject
+          end
         end
 
         context "with reception_status_event provided for return item" do