fix(typeahead): navigation buttons don't submit form anymore

Closes #693

src/typeahead/typeahead-window.spec.ts

@@ -129,6 +129,17 @@ describe('ngb-typeahead-window', () => {
       expectResults(fixture.nativeElement, ['bar', '+baz']);
       expect(fixture.componentInstance.active).toBe('baz');
     });
+
+    it('should have buttons of type button', () => {
+      const html = `
+           <ngb-typeahead-window [results]="results" [term]="term"></ngb-typeahead-window>`;
+      const fixture = createTestComponent(html);
+      const buttons = fixture.nativeElement.querySelectorAll('button');
+      expect(buttons.length).toBeGreaterThan(0);
+      for (let i = 0; i < buttons.length; i++) {
+        expect(buttons[i].getAttribute('type')).toBe('button');
+      }
+    });
   });
 
 });

src/typeahead/typeahead-window.ts

@@ -26,7 +26,7 @@ export interface ResultTemplateContext {
       <ngb-highlight [result]="formatter(result)" [term]="term"></ngb-highlight>
     </template>
     <template ngFor [ngForOf]="results" let-result let-idx="index">
-      <button class="dropdown-item" [class.active]="idx === activeIdx" 
+      <button type="button" class="dropdown-item" [class.active]="idx === activeIdx" 
         (mouseenter)="markActive(idx)" 
         (click)="select(result)">
           <template [ngTemplateOutlet]="resultTemplate || rt"