Change content-type for json stream to application/x-json-stream.

Fixes #6974

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Instead of doing else can we do
```go
w.Header().Set("Content-Type", "application/json")
if job.GetenvBoo("lineDelim") {
    w.Header.Set()("Content-Type", "application/x-json-stream")
}