Fixed issue #857.

When saving a JPEG and specifying 'keep' for quality or subsampling,
if the source JPEG image is in grayscale mode, don't try to find the
subsampling of the source, because grayscale images don't have any
subsampling (it's only for color components).

For the moment the fix also ignores subsampling of CMYK JPEG because
currently Pillow doesn't support encoding JPEG in YCCK mode (and
subsampling doesn't make sense in CMYK, but Pillow permits saving CMYK
JPEG with subsampling, that's a bug). This fix pass those errors
silently, i.e. it doesn't raise an error when 'keep' is used but it's
not possible to keep the subsampling (because the image is grayscale
or CMYK). I think it's the proper behavior but I'm not sure.