System.Net.Sockets.UdpClient Hostname Resolution ignores DualMode flag

[nbaztec]

Issue
For a UdpClient instance with underlying Socket having DualMode enabled, the socket does not work with hostnames that return IPv4 addresses.

Steps to reproduce

• Set up a local hostname with IPv4 address - foobar.local
• Create a UdpClient with AddressFamily.InterNetworkV6
• Set sender.Client.DualMode = true, to enable IPv4 over IPv6
• Send a datagram to a hostname sender.BeginSendAsync(new byte[1], 1, "foobar.local", 9000)
• An exception is thrown that no valid IPv6 addresses could be found for foobar.local

Why it happens?
After the DNS resolution for the hostname is done, the addresses are directly matched against the socket's family(InterAddressV6 in our case), completely ignoring DualMode operation.

I'll submit a PR shortly