Update hostpool.go

the preview code would return len(p.hosts) empty string first,then the true hosts.

hostpool.go

@@ -184,7 +184,7 @@ func (p *standardHostPool) markFailed(hostR HostPoolResponse) {
 
 }
 func (p *standardHostPool) Hosts() []string {
-	hosts := make([]string, len(p.hosts))
+	hosts := make([]string, 0, len(p.hosts))
 	for host := range p.hosts {
 		hosts = append(hosts, host)
 	}