Adjust logic of Web Cmdlet Authorization Header Stripping to work with .NET Core 2.1 #6728

Re #6718 #6632

In dotnet/corefx#26864 the Authorization header is now stripped on redirect. This means that the logic we currently have in place to handle this and to bypass is now reversed. We need to update the logic of the web cmdlets to manually handle redirection when -PreserveAuthorizationOnRedirect is supplied and an Authorization request header is present. Currently, they are doing the exact opposite.