Check content type along with content length

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Upstream-commit: 8266c381d62a0790c489e27cc93ed8f4618d03c7
Component: engine

components/engine/api/server/server.go

@@ -724,15 +724,16 @@ func postContainersStart(eng *engine.Engine, version version.Version, w http.Res
 	)
 
 	// allow a nil body for backwards compatibility
-	if r.Body != nil {
+	if r.Body != nil && r.ContentLength > 0 {
 		if !api.MatchesContentType(r.Header.Get("Content-Type"), "application/json") {
-			return fmt.Errorf("Content-Type is not supported: %s", r.Header.Get("Content-Type"))
+			return fmt.Errorf("Content-Type of application/json is required")
 		}
 
 		if err := job.DecodeEnv(r.Body); err != nil {
 			return err
 		}
 	}
+
 	if err := job.Run(); err != nil {
 		if err.Error() == "Container already started" {
 			w.WriteHeader(http.StatusNotModified)