fix(autocomplete): don't prevent default enter action if panel is closed

Currently the autocomplete will always prevented the default enter key action, even if the panel is closed, which goes against the native behavior.

Fixes angular#5976.

src/lib/autocomplete/autocomplete-trigger.ts

@@ -299,7 +299,7 @@ export class MdAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
   _handleKeydown(event: KeyboardEvent): void {
     if (event.keyCode === ESCAPE && this.panelOpen) {
       this.closePanel();
-    } else if (this.activeOption && event.keyCode === ENTER) {
+    } else if (this.activeOption && event.keyCode === ENTER && this.panelOpen) {
       this.activeOption._selectViaInteraction();
       event.preventDefault();
     } else {

src/lib/autocomplete/autocomplete.spec.ts

@@ -688,15 +688,28 @@ describe('MdAutocomplete', () => {
         fixture.componentInstance.trigger._handleKeydown(DOWN_ARROW_EVENT);
 
         fixture.whenStable().then(() => {
-          spyOn(ENTER_EVENT, 'preventDefault');
-
           fixture.componentInstance.trigger._handleKeydown(ENTER_EVENT);
 
-          expect(ENTER_EVENT.preventDefault).toHaveBeenCalled();
+          expect(ENTER_EVENT.defaultPrevented)
+              .toBe(true, 'Expected the default action to have been prevented.');
         });
       });
     }));
 
+    it('should not prevent the default enter action for a closed panel after a user interaction',
+      fakeAsync(() => {
+        tick();
+        fixture.componentInstance.trigger._handleKeydown(UP_ARROW_EVENT);
+        tick();
+        fixture.detectChanges();
+
+        fixture.componentInstance.trigger.closePanel();
+        fixture.detectChanges();
+        fixture.componentInstance.trigger._handleKeydown(ENTER_EVENT);
+
+        expect(ENTER_EVENT.defaultPrevented).toBe(false, 'Default action should not be prevented.');
+      }));
+
     it('should fill the text field, not select an option, when SPACE is entered', async(() => {
       fixture.whenStable().then(() => {
         typeInElement('New', input);