cvtColor: Fixed range for 'v' channel for 8U images

Fixed range for 'v' channel: 262 instead of 256.
Thanks to Sergey Perepelkin for reporting this issue.
Merge with extra: Itseez/opencv_extra#210

check_regression=*CvtMode*
test_filter=*CvtMode*
test_modules=imgproc
build_examples=OFF