plugin/forward: check TC correctly on reply. (#1670)

* plugin/forward: check TC correctly on reply.

Add test for this.

* Add proxy test as well

plugin/forward/truncated.go

@@ -1,10 +1,14 @@
 package forward
 
-import "github.com/miekg/dns"
+import (
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
 
 // truncated looks at the error and if truncated return a nil errror
 // and a possible reconstructed dns message if that was nil.
-func truncated(ret *dns.Msg, err error) (*dns.Msg, error) {
+func truncated(state request.Request, ret *dns.Msg, err error) (*dns.Msg, error) {
 	// If you query for instance ANY isc.org; you get a truncated query back which miekg/dns fails to unpack
 	// because the RRs are not finished. The returned message can be useful or useless. Return the original
 	// query with some header bits set that they should retry with TCP.
@@ -16,7 +20,7 @@ func truncated(ret *dns.Msg, err error) (*dns.Msg, error) {
 	m := ret
 	if ret == nil {
 		m = new(dns.Msg)
-		m.SetReply(ret)
+		m.SetReply(state.Req)
 		m.Truncated = true
 		m.Authoritative = true
 		m.Rcode = dns.RcodeSuccess