Add test for providers which don't support namespaces

A failing unit test was added to reproduce issue #39, the test
was then made to pass by handling the 404 error we know to be
thrown when the /system/namespaces endpoint is missing.

Fixes: #39

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

types/function_list_builder.go

@@ -48,14 +48,16 @@ func (s *FunctionLookupBuilder) getNamespaces() ([]string, error) {
 		defer res.Body.Close()
 	}
 
-	bytesOut, err := ioutil.ReadAll(res.Body)
-	if err != nil {
-		return namespaces, err
-	}
+	if res.StatusCode != http.StatusNotFound {
+		bytesOut, err := ioutil.ReadAll(res.Body)
+		if err != nil {
+			return namespaces, err
+		}
 
-	err = json.Unmarshal(bytesOut, &namespaces)
-	if err != nil {
-		return namespaces, err
+		err = json.Unmarshal(bytesOut, &namespaces)
+		if err != nil {
+			return namespaces, err
+		}
 	}
 
 	return namespaces, err

types/function_list_builder_test.go

@@ -392,6 +392,27 @@ func Test_GetNamespaces(t *testing.T) {
 	}
 }
 
+func Test_GetNamespaces_ProviderGives404(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		w.Write([]byte("Not available"))
+	}))
+
+	client := srv.Client()
+	builder := FunctionLookupBuilder{
+		Client:     client,
+		GatewayURL: srv.URL,
+	}
+
+	namespaces, err := builder.getNamespaces()
+	if err != nil {
+		t.Errorf("%s", err.Error())
+	}
+	if len(namespaces) != 0 {
+		t.Errorf("Namespaces - want: %d, got: %d", 2, len(namespaces))
+	}
+}
+
 func Test_GetFunctions(t *testing.T) {
 
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {