HTTP Basic Auth is ignored

## System info

Issue occurs on: both
Plugin version: 0.3.10+1
Flutter doctor output:

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, v1.12.13+hotfix.5, on Mac OS X 10.15.2 19C57, locale
en-US)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
[✓] Xcode - develop for iOS and macOS (Xcode 11.3.1)
[✓] Android Studio (version 3.5)
[✓] IntelliJ IDEA Community Edition (version 2019.2.3)
[✓] VS Code (version 1.39.1)
[✓] Connected device (1 available)

• No issues found!
```

## Steps to Reproduce
Point the webview at a url or hit a link to a url in a webview that requires the [HTTP authentication using the Basic schema](https://developer.mozilla.org/en-US/docs/Web/HTTP/Authentication). It will not prompt for login and will just be unauthorized

首先，使用需要 HTTP Basic 身份验证的 URL 在 Android 和 iOS 上分别复现该问题，然后检查插件原生 WebView 的身份验证处理。完成的标准是 webview 提示输入凭据，并成功加载受保护的 URL，而不是返回 unauthorized。

由索引模型根据 Issue 内容生成。