fix(service provider): change/fix return of preferredLanguage()

`$translateProvider.preferredLanguage()` was not returning the preferredLanguage
when used as an getter just like other methods. This is a fix.

As a result, this allows getting the preferred language during configuration phase.

Relates #1165

src/service/translate.js

@@ -391,12 +391,13 @@ function $translate($STORAGE_KEY, $windowProvider, $translateSanitizationProvide
    * only that it says which language to **prefer**.
    *
    * @param {string} langKey A language key.
-   *
    */
   this.preferredLanguage = function(langKey) {
-    setupPreferredLanguage(langKey);
-    return this;
-
+    if (langKey) {
+      setupPreferredLanguage(langKey);
+      return this;
+    }
+    return $preferredLanguage;
   };
   var setupPreferredLanguage = function (langKey) {
     if (langKey) {

test/unit/service/translate.spec.js

@@ -864,6 +864,23 @@ describe('pascalprecht.translate', function () {
     });
   });
 
+  describe('$translateProvider#preferredLanguage()', function () {
+    var translateProvider;
+
+    beforeEach(module('pascalprecht.translate', function($translateProvider) {
+      translateProvider = $translateProvider;
+      translateProvider.preferredLanguage('de_DE');
+    }));
+
+    it('should return a string if language is specified', inject(function () {
+      expect(typeof translateProvider.preferredLanguage()).toBe('string');
+    }));
+
+    it('should return a correct language code', inject(function () {
+      expect(translateProvider.preferredLanguage()).toEqual('de_DE');
+    }));
+  });
+
   describe('$translate#fallbackLanguage()', function () {
 
     describe('single fallback language', function () {