desktop_webview_window: HEAD/trunk version of WebView always generates GET requests (rather than POST etc...)

[TomKrauss]

For an expected POST request WebView execute all requests (on Windows) as a GET request. This happens only in the HEAD version of the plugin and may be caused by a regression introduced with the fix of #296.

Reproduce Steps

Steps to reproduce the behavior:

1. Go to login.microsoft.com (open the developer tools in web view)
2. Click on the key saying "Login Options" and then on "Security Key" (the developer tools should show a POST request, but a GET request is sent)

Expected behavior

a POST request should be fired in that case.

Version:

• Flutter Version: 3.19.3
• OS: Windows
• plugin: desktop_webview_window HEAD / trunk version