fix(datepicker): datepicker-popup nest in dropdown

- Add `dropdown-nested` class hack to avoid problems with dropdown
  controller

Closes #4489
Fixes #4197

Author: sullivanpt

src/dropdown/dropdown.js

@@ -250,7 +250,7 @@ angular.module('ui.bootstrap.dropdown', ['ui.bootstrap.position'])
     restrict: 'AC',
     require: '?^uibDropdown',
     link: function(scope, element, attrs, dropdownCtrl) {
-      if (!dropdownCtrl) {
+      if (!dropdownCtrl || angular.isDefined(attrs.dropdownNested)) {
         return;
       }
 

template/datepicker/popup.html

@@ -1,4 +1,4 @@
-<ul class="dropdown-menu" ng-if="isOpen" style="display: block" ng-style="{top: position.top+'px', left: position.left+'px'}" ng-keydown="keydown($event)" ng-click="$event.stopPropagation()">
+<ul class="dropdown-menu" dropdown-nested ng-if="isOpen" style="display: block" ng-style="{top: position.top+'px', left: position.left+'px'}" ng-keydown="keydown($event)" ng-click="$event.stopPropagation()">
 	<li ng-transclude></li>
 	<li ng-if="showButtonBar" style="padding:10px 9px 2px">
 		<span class="btn-group pull-left">