Change the single in flight key for Client.Exchange (#943)

Previously it was possible for two different questions to hit the same
single in flight entry if the type or class isn't in the relevant
XToString map. This could happen for a proxy server or similar.

client.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/binary"
+	"fmt"
 	"io"
 	"net"
 	"strings"
@@ -128,20 +129,15 @@ func (c *Client) Exchange(m *Msg, address string) (r *Msg, rtt time.Duration, er
 		return c.exchange(m, address)
 	}
 
-	t := "nop"
-	if t1, ok := TypeToString[m.Question[0].Qtype]; ok {
-		t = t1
-	}
-	cl := "nop"
-	if cl1, ok := ClassToString[m.Question[0].Qclass]; ok {
-		cl = cl1
-	}
-	r, rtt, err, shared := c.group.Do(m.Question[0].Name+t+cl, func() (*Msg, time.Duration, error) {
+	q := m.Question[0]
+	key := fmt.Sprintf("%s:%d:%d", q.Name, q.Qtype, q.Qclass)
+	r, rtt, err, shared := c.group.Do(key, func() (*Msg, time.Duration, error) {
 		return c.exchange(m, address)
 	})
 	if r != nil && shared {
 		r = r.Copy()
 	}
+
 	return r, rtt, err
 }