Merge pull request angular-ui#1908 of angular-ui/ui-select

[why] If selection is clear, the value is better to be null.
[how] Merge pull request:
      angular-ui#1908

Author: Rick Yang

src/uiSelectController.js

@@ -462,7 +462,7 @@ uis.controller('uiSelectCtrl',
   };
 
   ctrl.clear = function($event) {
-    ctrl.select(undefined);
+    ctrl.select(null);
     $event.stopPropagation();
     $timeout(function() {
       ctrl.focusser[0].focus();

test/select.spec.js

@@ -549,7 +549,7 @@ describe('ui-select tests', function() {
 
     // Trigger clear.
     el.find('.select2-search-choice-close').click();
-    expect(scope.selection.selected).toEqual(undefined);
+    expect(scope.selection.selected).toEqual(null);
 
     // If there is no selection it the X icon should be gone.
     expect(el.find('.select2-search-choice-close').length).toEqual(0);