proxy: use strings.TrimSuffix

Change-Id: Icca4cdbdc1fb964eda8e0fed559f8d4e5dc45073
Reviewed-on: https://go-review.googlesource.com/c/net/+/586115
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: tklauser

proxy/per_host.go

@@ -137,9 +137,7 @@ func (p *PerHost) AddNetwork(net *net.IPNet) {
 // AddZone specifies a DNS suffix that will use the bypass proxy. A zone of
 // "example.com" matches "example.com" and all of its subdomains.
 func (p *PerHost) AddZone(zone string) {
-	if strings.HasSuffix(zone, ".") {
-		zone = zone[:len(zone)-1]
-	}
+	zone = strings.TrimSuffix(zone, ".")
 	if !strings.HasPrefix(zone, ".") {
 		zone = "." + zone
 	}
@@ -148,8 +146,6 @@ func (p *PerHost) AddZone(zone string) {
 
 // AddHost specifies a host name that will use the bypass proxy.
 func (p *PerHost) AddHost(host string) {
-	if strings.HasSuffix(host, ".") {
-		host = host[:len(host)-1]
-	}
+	host = strings.TrimSuffix(host, ".")
 	p.bypassHosts = append(p.bypassHosts, host)
 }