feat(ui-i18n.js): Allow replacing/customizing specific existing locale strings when using add.

Use `merge` instead of `extend`. Extending will *replace* the *entire* existing object with the new one. Merging will *add* new strings into existing objects, thereby preserving any strings that are set elsewhere in code.

Author: prettycode

src/js/i18n/ui-i18n.js

@@ -61,7 +61,7 @@
           if (!this._langs[lower]) {
             this._langs[lower] = {};
           }
-          angular.extend(this._langs[lower], strings);
+          angular.merge(this._langs[lower], strings);
         },
         getAllLangs: function () {
           var langs = [];
@@ -301,4 +301,4 @@
   } );
 
 
-})();
+})();