fix content-type for job.Stdout.Add

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
fix content-type for legacy

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #4128 from vieux/fix_header_content-type_api

Fix header content type api