Passing proxy info along when registering default repository

[andrewaramsay]

I'm not sure if this was just an oversight or if it was intentionally left out, but if I specify -Proxy and/or -ProxyCredentials when calling Register-PSRepository -Default, they get ignored because they don't get passed along here. In PowerShell 6 this makes it impossible to register PS Gallery behind a proxy, since you can't set the default proxy on HttpClient.

I'm not super familiar with PowerShell testing, but it didn't look like we were doing mock/expect type testing, and setting a proxy value for real would always fail unless there was in fact a proxy running. If there is a place/approach I should use to test this, let me know and I'll try to get it added.