fix(datepicker): ensure the original target is not in popup

- Ensure that the original event target is not a child of the popup element to avoid edge case where the datepicker closes on any click

Closes #4316
Fixes #4314

Author: wesleycho

Diff for: src/datepicker/datepicker.js

@@ -715,7 +715,7 @@ function($compile, $parse, $document, $rootScope, $position, dateFilter, datePar
       });
 
       var documentClickBind = function(event) {
-        if (scope.isOpen && !element[0].contains(event.target)) {
+        if (scope.isOpen && !(element[0].contains(event.target) || popupEl[0].contains(event.target))) {
           scope.$apply(function() {
             scope.isOpen = false;
           });