Make JPEG encoder output valid images again.
Make JPEG encoder output valid images again.

Fixes issue #618. It looks like commit 2c06a405ac7e3a04d139c1de16d07263c4a13573
stopped emitting the DC table by accident, resulting in JPEG files missing
huffman table 0.

PistonDevelopers/image,PistonDevelopers/image,nwin/rust-image,mbrubeck/image