Regression: Content-Type header not returned anymore

Hello,

We just upgraded today to httpclient5 version 5.4. It seems that the header `Content-Type` is not returned anymore by the API, even when listing all the headers.

client.execute(httpRequest, r -> {
    //does not print Content-Type header 
    for(Header h : r.getHeaders()) {
        System.out.println("## "+h.getName()+": "+h.getValue());
    }
    System.out.flush();
    return null;
});

We can ensure that the `Content-Type` header is well set by the server by browsing to the same URL and inspecting the network.

Our code didn't change and with the version 5.3.1, the Content-Type is correctly received.