fix(select): ensure that at least one option has the selected attribute set

Using `prop` to set selected is correct programmatically but accessibility
guidelines suggest that at least on item should have the `selected` attribute
set.

Closes #8366
Closes #8429

Author: petebacondarwin

src/ng/directive/select.js

@@ -576,6 +576,7 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
                   (element = optionTemplate.clone())
                       .val(option.id)
                       .prop('selected', option.selected)
+                      .attr('selected', option.selected)
                       .text(option.label);
                 }
 

test/ng/directive/selectSpec.js

@@ -1008,6 +1008,47 @@ describe('select', function() {
 
         expect(scope.selected).toBe(scope.values[1]);
       });
+
+
+      it('should ensure that at least one option element has the "selected" attribute', function() {
+        createSelect({
+          'ng-model': 'selected',
+          'ng-options': 'item.id as item.name for item in values'
+        });
+
+        scope.$apply(function() {
+          scope.values = [{id: 10, name: 'A'}, {id: 20, name: 'B'}];
+        });
+        expect(element.val()).toEqual('?');
+        expect(element.find('option').eq(0).attr('selected')).toEqual('selected');
+
+        scope.$apply(function() {
+          scope.selected = 10;
+        });
+        // Here the ? option should disappear and the first real option should have selected attribute
+        expect(element.val()).toEqual('0');
+        expect(element.find('option').eq(0).attr('selected')).toEqual('selected');
+
+        // Here the selected value is changed but we don't change the selected attribute
+        scope.$apply(function() {
+          scope.selected = 20;
+        });
+        expect(element.val()).toEqual('1');
+        expect(element.find('option').eq(0).attr('selected')).toEqual('selected');
+
+        scope.$apply(function() {
+          scope.values.push({id: 30, name: 'C'});
+        });
+        expect(element.val()).toEqual('1');
+        expect(element.find('option').eq(0).attr('selected')).toEqual('selected');
+
+        // Here the ? option should reappear and have selected attribute
+        scope.$apply(function() {
+          scope.selected = undefined;
+        });
+        expect(element.val()).toEqual('?');
+        expect(element.find('option').eq(0).attr('selected')).toEqual('selected');
+      });
     });