Cache cannot be disabled or clear on WebEngineView

Using the following QML, the cache is still activated and not cleared:

```
import QtQuick 2.2
import QtQuick.Window 2.2
import QtWebEngine 1.5

WebEngineView {
  id: webview
  visible: true
  url: context.getUrl()
  width: Window.width
  height: Window.height
  profile.httpCacheType: WebEngineProfile.NoCache

  onLoadingChanged: {
    if (loadRequest.errorString)

}

  Connections {
    target: context
    onUrlChanged:

  }
}
```
Visiting: http://refreshyourcache.com/en/cache-test/
the image stays red.

The site works properly on Chromium when "disable cache" is enabled.