image/jpeg: encode grayscale images as grayscale JPEGs.

[nigeltao]

The jpeg.Encode function always writes YCbCr JPEGs, even if the source image is
grayscale.

[wathiede]

Comment 1:

I sent https://golang.org/cl/105990046 as one potential solution for this
issue.  Not sure if you were planning to work on the issue.  If you are, I can abandon
my CL.
https://golang.org/cl/105990046 allows the test in
https://camlistore.org/r/1217 to exercise the PNM type 5 code path.

[nigeltao]

Comment 2:

This issue was closed by revision 57964db.

Status changed to Fixed.