Only render layout on first call

This updates the logic around rendering a layout when `render` is called and a default layout is set.

Since multiple calls to render from a Controller are unnecessary, there should be no need to return `@_no_default_layout` to false again, and currently returning the value to `true` means that additional calls to `render` inside a view will start rendering the default layout again.

Although I locally updated the specs and tested it, I didn't include that since there is already a test about rendering inside a view. It was just a matter of updating `composer.erb` and rendering the partial twice to test.