Convert path to string to avoid Graphiti::Errors::InvalidLink

The `path` argument to the method containing the line below is often a symbol, which causes the call to fail, eventually resulting in Graphiti raising `Graphiti::Errors::InvalidLink`. I suggest adding `.to_s` to `path` here.

https://github.com/graphiti-api/graphiti-rails/blame/759db8d25f14ae4b171e9b961e51ab50027a0d67/lib/graphiti/rails/railtie.rb#L134