Improve the condition whether NetWorkCredential is valid or not

This PR is new version of the PR #332 .

`[net.CredentialCache]::DefaultCredentials` may return a default `System.Net.NetworkCredential` instance.
But Chocolatey checks only if `$cred` is null or not for the need of NetworkCredential.
It may have another check whether this variable is the default NetworkCredential instance or not.