Fix HttpWebRequest when using system proxy settings (dotnet/corefx#31100)

Fix HttpWebRequest when using system proxy settings (dotnet/corefx#31100)

While investigating other HttpClient/HttpWebRequest proxy-related bugs, I discovered that
HttpWebRequest was not honoring system proxy settings as defined on Windows with IE
settings or on Linux using environment variables.

The problem is due to how HttpClient and HttpWebRequest differ in how they represent
the default behavior of using system proxy settings with the various properties. Fixed
HttpWebRequest so that it will translate the system proxy settings to the internal
HttpClient/HttpClientHandler objects.

I also removed an invalid Assert in HttpConnection. This assert was firing when using a proxy
that was defined on the loopback adapter using IPv6 literal "[::1]".  Due to issue dotnet/corefx#28863 with Uri,
the Uri.IdnHost property doesn't have the brackets for IPv6 literals. So, the Assert was
occuring.

I did not add any new CI tests because it is currently not possible to test system proxy settings
in CI since it involves changing machine configuration. But I ran manual tests.

Commit migrated from https://github.com/dotnet/corefx/commit/72119235e294e711871cf453f2ffc66ba17ff23b