Don't use public_send in destroy_conditionally! helper

As we only override in two places, we could just ask for the value rather than
the method name.

lib/api/branches.rb

@@ -130,7 +130,7 @@ module API
 
         commit = user_project.repository.commit(branch.dereferenced_target)
 
-        destroy_conditionally!(commit, last_update_field: :authored_date) do
+        destroy_conditionally!(commit, last_updated: commit.authored_date) do
           result = DeleteBranchService.new(user_project, current_user)
                     .execute(params[:branch])
 

lib/api/helpers.rb

@@ -19,8 +19,10 @@ module API
       end
     end
 
-    def destroy_conditionally!(resource, last_update_field: :updated_at)
-      check_unmodified_since!(resource.public_send(last_update_field))
+    def destroy_conditionally!(resource, last_updated: nil)
+      last_updated ||= resource.updated_at
+
+      check_unmodified_since!(last_updated)
 
       status 204
       if block_given?

lib/api/tags.rb

@@ -70,7 +70,7 @@ module API
 
         commit = user_project.repository.commit(tag.dereferenced_target)
 
-        destroy_conditionally!(commit, last_update_field: :authored_date) do
+        destroy_conditionally!(commit, last_updated: commit.authored_date) do
           result = ::Tags::DestroyService.new(user_project, current_user)
                     .execute(params[:tag_name])