Switch to deep_merge for adding included_objects

as found in #802

Author: lgebhardt

lib/jsonapi/resource_serializer.rb

@@ -348,7 +348,7 @@ def add_included_object(id, object_hash, primary = false)
       @included_objects[type] = {} unless @included_objects.key?(type)
 
       if already_serialized?(type, id)
-        @included_objects[type][id][:object_hash].merge!(object_hash)
+        @included_objects[type][id][:object_hash].deep_merge!(object_hash)
         set_primary(type, id) if primary
       else
         @included_objects[type].store(id, primary: primary, object_hash: object_hash)