resolveHost: Check expected address family on returned address

The code as is never checks the requested address family when doing a DNS query,
so resolving an IPv6 only host with AddressFamily.INET would still return an IPv6.

tests/pull-218-resolvehost-dns-address-family.d

@@ -0,0 +1,39 @@
+/+ dub.sdl:
+    name "test"
+    dependency "vibe-core" path=".."
++/
+module test;
+
+import std.socket: AddressFamily;
+
+import vibe.core.core;
+import vibe.core.net;
+
+void main()
+{
+    runTask({
+        scope(exit) exitEventLoop();
+
+        auto addr = resolveHost("ip6.me", AddressFamily.INET);
+        assert(addr.family == AddressFamily.INET);
+
+        addr = resolveHost("ip6.me", AddressFamily.INET6);
+        assert(addr.family == AddressFamily.INET6);
+
+        try
+        {
+            resolveHost("ip4only.me", AddressFamily.INET6);
+            assert(false);
+        }
+        catch(Exception) {}
+
+        try
+        {
+            resolveHost("ip6only.me", AddressFamily.INET);
+            assert(false);
+        }
+        catch(Exception) {}
+    });
+
+    runEventLoop();
+}