Update for scaling edge-case

- as reported on Slack and in issue #931 the gateway scaling code
was scaling to zero replicas as a result of the "proportional
scaling" added by @Templum's PR. This commit added a failing test
which was fixed by adding boundary checking - now if the scaling
amount is "0" we keep the current amount of replicas.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/handlers/alerthandler.go

@@ -5,12 +5,11 @@ package handlers
 
 import (
 	"encoding/json"
+	"fmt"
 	"io/ioutil"
 	"log"
 	"net/http"
 
-	"fmt"
-
 	"github.com/openfaas/faas/gateway/requests"
 )
 
@@ -119,7 +118,7 @@ func CalculateReplicas(status string, currentReplicas uint64, maxReplicas uint64
 	newReplicas := currentReplicas
 	step := uint64((float64(maxReplicas) / 100) * float64(scalingFactor))
 
-	if status == "firing" {
+	if status == "firing" && step > 0 {
 		if currentReplicas == 1 {
 			newReplicas = step
 		} else {