return 304 is status isn't modified in start and stop

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
fix tests

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6649 from vieux/fix_api_return_codes_start_stop

return 304 is status isn't modified in start and stop