DCTDecode image with separation colorspace not rendered correctly

PDF reported by Ford Wang in the mailing list. The "herbs" have weird purple dots. The cause are DCTDecode (jpeg) images with a separation colorspace, e.g. Root/Pages/Kids/[0]/Resources/XObject/Im5267 . The image itself is a grayscale image, i.e. it has only one color component. The separation colorspace is a green (PANTONE 355 C), which is based on an Lab colorspace.