fix(dropdown): do not close on right click

- Do not close the dropdown on right click when open due to Firefox's
  behavior

Closes #5052
Fixes #5051

Author: wesleycho

src/dropdown/dropdown.js

@@ -36,6 +36,8 @@ angular.module('ui.bootstrap.dropdown', ['ui.bootstrap.position'])
 
     if (evt && openScope.getAutoClose() === 'disabled') { return; }
 
+    if (evt && evt.which === 3) { return; }
+
     var toggleElement = openScope.getToggleElement();
     if (evt && toggleElement && toggleElement[0].contains(evt.target)) {
       return;

src/dropdown/test/dropdown.spec.js

@@ -79,6 +79,15 @@ describe('uib-dropdown', function() {
       expect(element).toHaveClass(dropdownConfig.openClass);
     });
 
+    it('should not close on right click', function() {
+      clickDropdownToggle();
+      element.find('ul a').trigger({
+        type: 'mousedown',
+        which: 3
+      });
+      expect(element).toHaveClass(dropdownConfig.openClass);
+    });
+
     it('should close on $location change', function() {
       clickDropdownToggle();
       expect(element).toHaveClass(dropdownConfig.openClass);