fix getting default rpc host error when access AriaNg by opening file directly

src/scripts/config/constants.js

@@ -9,6 +9,7 @@
         languagePath: '/langs',
         languageFileExtension: '.json',
         defaultLanguage: 'en',
+        defaultHost: 'localhost',
         globalStatStorageCapacity: 120,
         taskStatStorageCapacity: 300,
         lazySaveTimeout: 500,

src/scripts/services/ariaNgSettingService.js

@@ -32,7 +32,13 @@
         };
 
         var getDefaultRpcHost = function () {
-            return $location.$$host;
+            var currentHost = $location.$$host;
+
+            if (currentHost) {
+                return currentHost;
+            }
+
+            return ariaNgConstants.defaultHost;
         };
 
         var setOptions = function (options) {