fix(select): remove unknown option when model is undefined and empty option is available

Closes angular#11078
Closes angular#11092

Author: petebacondarwin

src/ng/directive/select.js

@@ -73,6 +73,7 @@ var SelectController =
       if (value === '') self.emptyOption.prop('selected', true); // to make IE9 happy
     } else {
       if (isUndefined(value) && self.emptyOption) {
+        self.removeUnknownOption();
         $element.val('');
       } else {
         self.renderUnknownOption(value);

test/ng/directive/selectSpec.js

@@ -237,6 +237,23 @@ describe('select', function() {
       });
 
 
+      it('should remove unknown option when model is undefined', function() {
+        scope.robot = 'other';
+        compile('<select ng-model="robot">' +
+                  '<option value="">--select--</option>' +
+                  '<option value="x">robot x</option>' +
+                  '<option value="y">robot y</option>' +
+                '</select>');
+
+        expect(element).toEqualSelect([unknownValue('other')], '', 'x', 'y');
+
+        scope.robot = undefined;
+        scope.$digest();
+
+        expect(element).toEqualSelect([''], 'x', 'y');
+      });
+
+
       describe('interactions with repeated options', function() {
 
         it('should select empty option when model is undefined', function() {