If you try to load an URL into the WebView where the response does not specify the content-type, the Loader state switch to "cancelled". 

STEPS TO REPRODUCE (a bit technical because we want to remove headers):

- Download Fiddler here : https://www.telerik.com/download/fiddler
- Launch it, then click on "Rules" -> "Customize rules"
- On the Fiddler script editor, scroll to the function "OnBeforeResponse" and modify it like that :

static function OnBeforeResponse(oSession: Session) {
        if (m_Hide304s && oSession.responseCode == 304) {
            oSession["ui-hide"] = "true";
        }
		if (oSession.uriContains("google") && oSession.responseCode == 200) {
			oSession["ui-color"] = "red";
			oSession.oResponse.headers.Remove("Content-Type");
		}
    }

- Then run the class given in attachment

EXPECTED RESULT:
The WebView should load the google webPage and display it.

ACTUAL RESULT: 
The WebView loader switch to "Cancelled" state.

EXPLANATION:

If we look at the RFC : https://tools.ietf.org/html/rfc7231#section-3.1.1.5 , we can see that the Content-type is not mandatory. Browser do display properly a webPage without Content-Type.

If we debug, we can see that in com.sun.webkit.network.URLLoader, we enter the method "notifyDidReceiveResponse", then call the native method "twkDidReceiveResponse". 
I do not have access to the native code, but after that, the WebView is notified to switch to Cancelled mode because the response is detected as incorrect.

WORK-AROUND:
Currently, it's a bit difficult. A solution consists of providing a custom URLStreamHandlerFactory in URL class. Then we can provide a custom Handler that would override "openConnection" in order to retrieve the URLConnection, wrap it and delegates all methods except from "getContentType" in order to return a default content-type when null.