host[all]: fix #114 String() issue.

host/host_test.go

@@ -46,8 +46,9 @@ func TestHostInfoStat_String(t *testing.T) {
 		Procs:    100,
 		OS:       "linux",
 		Platform: "ubuntu",
+		BootTime: 1447040000,
 	}
-	e := `{"hostname":"test","uptime":3000,"procs":100,"os":"linux","platform":"ubuntu","platform_family":"","platform_version":"","virtualization_system":"","virtualization_role":""}`
+	e := `{"hostname":"test","uptime":3000,"boot_time":1447040000,"procs":100,"os":"linux","platform":"ubuntu","platform_family":"","platform_version":"","virtualization_system":"","virtualization_role":""}`
 	if e != fmt.Sprintf("%v", v) {
 		t.Errorf("HostInfoStat string is invalid: %v", v)
 	}