Remove `Content-Length` header for valid JSON response

Set content to `None` so the response does not set a `Content-Length`
header with value 0, which would tell clients to parse the body as JSON.
An empty string (response body with length 0) is invalid JSON.

Response before this change:

    HTTP/1.1 201 Created
    Location: http://localhost:8080/v2/apps/app-1234
    Content-Type: application/json
    Content-Length: 0
    Server: Jetty(8.y.z-SNAPSHOT)

Response after this change:

    HTTP/1.1 201 Created
    Location: http://localhost:8080/v2/apps/app-12345
    Content-Type: application/json
    Transfer-Encoding: chunked
    Server: Jetty(8.y.z-SNAPSHOT)

Fixes #349.