Use super rather than render_all in single block render classes.

@Shopify/liquid for review

Block#render does `render_all(@nodelist, context)` which we do in derived classes in the render method instead of calling `super`.  This is a bad practice, since we should try to keep code at a single layer of abstraction to avoid exposing the reader to implementation details in the base class.
or is it initialized in the Liquid::Block constructor?