Check for default function before attribute changes for partial inserts

We're limiting the attributes being used for INSERTs to those without a
default function and no changes. But we can first check for the presence
of a default function before checking for changes.

That's because the default function will be stored in the schema cache
and amounts to a hash fetch in column_for_attribute, while checking for
changes potentially calls user code when using the Active Record
Attributes API.

Author: etiennebarrie

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -233,7 +233,9 @@ def attribute_names_for_partial_inserts
             changed_attribute_names_to_save
           else
             attribute_names.reject do |attr_name|
-              !attribute_changed?(attr_name) && column_for_attribute(attr_name).default_function
+              if column_for_attribute(attr_name).default_function
+                !attribute_changed?(attr_name)
+              end
             end
           end
         end