bug in ofColor::setHsb() - unnecessary special cases for sat and brightness to zero

I wanted to use HSB for animating colors but with these exceptions in ofColor::setHsb this is harder than it should:

For example if you want to animate from blue to black and back to blue you normally only need to animate the brightness to 0 and back to 255. But with this implementation you will end up with white and not blue because of the two exceptions for saturation and brightness == 0.

Or in other words: Settings the brightness to zero doesn't mean black - it is just dark for the moment :)

I think it is the same for saturation. Turning the saturation to zero for a moment should not switch my color into grayscale.

Start in libs/openFrameworks/types/ofColor.cpp at the referenced ofColor::setHsb() exceptions around lines 469-472. Compare the current handling of zero saturation and brightness with the blue-to-black-to-blue and temporary-zero-saturation cases described in the issue. Done means the HSB animation behavior matches those expectations without the unnecessary special cases.