FIX: allow both meta_data and custom_fields when creating a topic

For some reasons, we have two ways of associating "custom fields" to a new topic:
using 'meta_data' and 'custom_fields'.

However, if we were to provide both arguments, the 'meta_data' would be overwritten
by any 'custom_fields' provided.

This commit ensures we can use both and merges the 'custom_fields' with the 'meta_data'.

lib/topic_creator.rb

@@ -35,8 +35,9 @@ class TopicCreator
   def create
     topic = Topic.new(setup_topic_params)
     setup_tags(topic)
+
     if fields = @opts[:custom_fields]
-      topic.custom_fields = fields
+      topic.custom_fields.merge!(fields)
     end
 
     DiscourseEvent.trigger(:before_create_topic, topic, self)