RGB.on() and .off() saves color state

Led.RGB.on() and Led.RGB.off() weren’t saving color state.

It was also treating the RGB as individual LEDs and calling Led.on()
was causing unexpected results because there isn’t the concept of
individual brightness on the RGB pins. (It just produced unexpected
color combinations rather than “dimming” them.)

This simplified implementation will need to be revisited when/if #375
is addressed, but at least it’s consistent and intuitive for now.
Led.RGB.on() is essentially the same as Led.RGB.color(“#ffffff”) with
this update.

Color also wasn’t being checked for in the shape tests for RGB (both
varieties)

Fixes #445
But `on` doesn't necessarily mean "white", it just means the opposite of `off`, if the color was set to red and I turn it off, turning it back on should restore it to that color. I definitely think that call to `this[color].on();` needs to be removed.