Add error handling to basic auth injector

Fixes a problem where basic auth was disabled and a nill pointer
was hit, causing a panic.

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

gateway/handlers/basic_auth_injector_test.go

@@ -0,0 +1,21 @@
+// Copyright (c) OpenFaaS Author(s). All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package handlers
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func Test_Inject_WithNilRequestAndNilCredentials(t *testing.T) {
+	injector := BasicAuthInjector{}
+	injector.Inject(nil)
+}
+
+func Test_Inject_WithRequestButNilCredentials(t *testing.T) {
+	injector := BasicAuthInjector{}
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	injector.Inject(req)
+}