WebViewScaffold seems to ignore provided headers

Hi,

when calling a URL and placing a cookie header in the webview scaffold, the request does not seem to pick up the provided header.
using: flutter_webview_plugin: ^0.3.4

```dart
@override
Widget build(BuildContext context) {
return WebviewScaffold (
url: this.url,
headers: {'Cookie:' : this.cookie},
appBar: AppBar(
title: Text('Inhalt', style: TextStyle(color: Colors.white)),
centerTitle: true,
backgroundColor: Colors.lightGreen,
),
);
}
```
When i am setting up a classic http.get command it works perfectly fine.
```dart
var headers = {'Cookie': this.cookie};
final response = await http.get(
'https://someurl.com',
headers: headers);
```
So the cookie seems to be fine, the header declaration also.
I could not find a more comprehensive documentation on this. Maybe I am missing something.

Thanks!