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.
It is initialized in [Block#parse](https://github.com/Shopify/liquid/blob/478eb893a9622cd2ae19f67208421ef44f0e4040/lib/liquid/block.rb#L14)