fix: 新密钥不传给cosApi

[toknT]

https://github.com/TencentCloud/cos-sdk-flutter-plugin/issues/13

强制清老临时密钥和用新的临时密钥,新密钥不传给cosApi

  /// 强制让本地保存临时秘钥失效
  /// 包括SessionCredential或ScopeLimitCredential
  Future<void> forceInvalidationCredential() async {
    flutterCosApi.forceInvalidationScopeCredentials();
    await _cosApi.forceInvalidationCredential();
    _initialized = false; // 加了这一步不知道合不合适
  }