image/jpeg: send a correct Start Of Scan (SOS) header.
Section B.2.3 of http://www.w3.org/Graphics/JPEG/itu-t81.pdf discusses
the End of spectral selection (Se) byte.
Apparently many JPEG decoders ignore the Se byte (or let it through
with a warning), but some configurations reject them. For example,
http://download.blender.org/source/chest/blender_2.03_tree/jpeg/jcmaster.c
has these lines:
if (Ss != 0 || Se != DCTSIZE2-1 || Ah != 0 || Al != 0)
ERREXIT1(cinfo, JERR_BAD_PROG_SCRIPT, scanno);
Fixes #3916.
R=r
CC=golang-dev
https://golang.org/cl/
6459052