ShadowWebView.getLastLoadedUrl() should the same url from WebView.loadUrl(url, additionalHeaders)

When we call WebView.loadUrl("http://www.google.com"), ShadowWebView.getLastLoadedUrl() will give correct result, "http://www.google.com". However, when we call WebView.loadUrl("http://www.google.com", null), ShadowWebView.getLastLoadedUrl() will return null, which is wrong. This patch will fix it.

This patch also provides a way to test the second parameter in the call WebView.loadUrl(url, additionalHttpHeaders)