Don't mark the schema loaded until it's really finished

load_schema! is overridden by attribute modules, so we need to wait
until it has returned.

activerecord/lib/active_record/model_schema.rb

@@ -459,7 +459,11 @@ def schema_loaded?
         def load_schema
           return if schema_loaded?
           @load_schema_monitor.synchronize do
-            load_schema! unless defined?(@columns_hash) && @columns_hash
+            return if defined?(@columns_hash) && @columns_hash
+
+            load_schema!
+
+            @schema_loaded = true
           end
         end
 
@@ -473,8 +477,6 @@ def load_schema!
               user_provided_default: false
             )
           end
-
-          @schema_loaded = true
         end
 
         def reload_schema_from_cache