fix(dropdown): add "dropdown" class for placements other than "top"

Fixes #1847
Closes #1852

src/dropdown/dropdown.spec.ts

@@ -83,6 +83,21 @@ describe('ngb-dropdown', () => {
     expect(getDropdownEl(compiled)).toHaveCssClass('dropup');
   });
 
+  it('should have dropdown CSS class if placement is other than top', () => {
+    const html = `
+      <div ngbDropdown placement="bottom">
+          <div ngbDropdownMenu>
+            <a class="dropdown-item">dropDown item</a>
+            <a class="dropdown-item">dropDown item</a>
+          </div>
+      </div>`;
+
+    const fixture = createTestComponent(html);
+    const compiled = fixture.nativeElement;
+
+    expect(getDropdownEl(compiled)).toHaveCssClass('dropdown');
+  });
+
   it('should be open initially if open expression is true', () => {
     const html = `
       <div ngbDropdown [open]="true">

src/dropdown/dropdown.ts

@@ -35,14 +35,16 @@ export class NgbDropdownMenu {
   applyPlacement(_placement: Placement) {
     // remove the current placement classes
     this._renderer.removeClass(this._elementRef.nativeElement.parentElement, 'dropup');
+    this._renderer.removeClass(this._elementRef.nativeElement.parentElement, 'dropdown');
     this.placement = _placement;
     /**
      * apply the new placement
-     * change the class only in case of top to show up arrow
-     * or use defualt which is dropdown to show down arrow
+     * in case of top use up-arrow or down-arrow otherwise
      */
     if (_placement.search('^top') !== -1) {
       this._renderer.addClass(this._elementRef.nativeElement.parentElement, 'dropup');
+    } else {
+      this._renderer.addClass(this._elementRef.nativeElement.parentElement, 'dropdown');
     }
   }
 }