Fix CI for versions < 7.2

Author: seanpdoyle

lib/active_resource/base.rb

@@ -1612,6 +1612,13 @@ def load(attributes, remove_root = false, persisted = false)
       self
     end
 
+    if ActiveModel::VERSION::MAJOR <= 7 && ActiveModel::VERSION::MINOR < 2
+      def assign_attributes(new_attributes)
+        new_attributes = new_attributes.stringify_keys if new_attributes
+        super
+      end
+    end
+
     # Updates a single attribute and then saves the object.
     #
     # Note: <tt>Unlike ActiveRecord::Base.update_attribute</tt>, this method <b>is</b>