Renderer.checkbox uses the checked keyword param

Right now, `Renderer.checkbox("name", checked=True)` will ignore the `checked` keyword param and always pass `False` to the `tags.checkbox` method, even if there is no value available for `"name"`. This change allows users to request the renderer to default `checked` to `True`, while still reflecting changes when data is available.

This behavior (allowing a renderer default) matches that of several existing methods, including `textarea`, `text`, and `password`.