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
>> After creating an RGB led instance, .color() returns { red: 0, green: 0, blue: 0 }
>> After a call to .on(), what should .color() return? { red: 255, green: 255, blue: 255 }?

> I would say yes to 255 across, but what do you think?

agreed

>> If I set .color("#ff0000") .color() should return { red: 255, green: 0, blue: 0 }

> What happens when isAnode is set? The inverse or do we normalize? I vote normalize, otherwise it will be really confusing to get the opposite of what you'd expect "blue" to be.

Normalize. "#0000ff" should be blue regardless. The current tests already assume this. I'll expand the tests to verify that a subsequent call to .color() returns the proper values.

>> After a call to .off(), what should .color() return? The same? or { red: 0, green: 0, blue: 0 }
>> i.e. does it return the "saved" previous color or the current state of the led?

> It should return the saved color.

Roger that. I agree. PR #447 doesn't do this yet because there is no mechanism for storing the "previous" color separate from the current state. I'll add that when I do a PR for #375.

I have a feeling I'll have more questions on the rest of those functions. For example, what does it mean to change the brightness of an RGB LED? Difficult to dim the bulb but keep the same color. But I'll address those questions when they come up in another issue.