fix(select): auto-select new option that is marked as selected

When adding a new <option> element, if the DOM of this option element
states that the element is marked as `selected`, then select the new
<option> element

Closes #6828

src/ng/directive/select.js

@@ -163,14 +163,17 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
       };
 
 
-      self.addOption = function(value) {
+      self.addOption = function(value, element) {
         assertNotHasOwnProperty(value, '"option value"');
         optionsMap[value] = true;
 
         if (ngModelCtrl.$viewValue == value) {
           $element.val(value);
           if (unknownOption.parent()) unknownOption.remove();
         }
+        if (element.attr('selected') === 'selected') {
+          element[0].selected = true;
+        }
       };
 
 
@@ -622,10 +625,10 @@ var optionDirective = ['$interpolate', function($interpolate) {
           scope.$watch(interpolateFn, function interpolateWatchAction(newVal, oldVal) {
             attr.$set('value', newVal);
             if (newVal !== oldVal) selectCtrl.removeOption(oldVal);
-            selectCtrl.addOption(newVal);
+            selectCtrl.addOption(newVal, element);
           });
         } else {
-          selectCtrl.addOption(attr.value);
+          selectCtrl.addOption(attr.value, element);
         }
 
         element.on('$destroy', function() {