onHttpError called with a 200 code

I'm using this plugin for a payment process (using a bank web interface).
I first load a generated HTML page (given a `Uri.dataFromString()` with `mimeType` sets to `text/html`) that build a form that auto-submit with action field sets to the bank server URL (where you enter your credit card information).

This workflow works fine on Android but I'm having a strange behavior on iOS. Right after the state changed to `WebViewState.startLoad` (with the bank server URL), `onHttpError` is triggered with a HTTP 200 code.

I initially quit the webview when an error occur to show a message. So I add a test to avoid false error codes and the web page works fine despite the error.

My app use the latest version (0.3.5).

I would like to know what can cause this behavior, I tried to put breakpoints in Xcode to have more info directly from the `WKWebView didFailNavigation` (inside FlutterWebviewPlugin.m) but my breakpoint is never hit...

**EDIT:** Putting the breakpoint in `WKWebView decidePolicyForNavigationResponse` works.
Reading the code inside this method I noticed that there is no test to determined if the response is effectively an error.

FlutterWebviewPlugin.m、特にレポートでブレークポイントに到達するとされている WKWebView decidePolicyForNavigationResponse から開始してください。onHttpError がトリガーされる前にナビゲーションレスポンスがどのように分類されるかを追跡し、HTTP 200 のレスポンスがエラーとして扱われないことを確認してください。その後も既存の WebView の動作が引き続き機能する必要があります。

索引モデルが issue の本文から書いたものです。