rgb_array bug: wrap arround of uint8 (#263)
rgb_array bug: wrap arround of uint8 (#263)

* rgb_array bug: wrap arround of uint8
enforce the cast to float before the subtraction (l350, 351)
to avoid wrap-around in the case of low-light images

* reset the memoized RGB array at flush

tfroehlich82/picamera,waveform80/picamera