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.