compact for multiple belongs_to suppo

Author: ollizeyen

lib/json_api_client/resource.rb

@@ -313,15 +313,15 @@ def _prefix_path
           a.to_prefix_path(route_formatter)
         end
 
-        paths.join("/")
+        paths.compact.join("/")
       end
 
       def _set_prefix_path(attrs)
         paths = _belongs_to_associations.map do |a|
           a.set_prefix_path(attrs, route_formatter)
         end
 
-        paths.join("/")
+        paths.compact.join("/")
       end
 
       def _new_scope