fix(datepicker): fix support for literal format on popup

- Add check for `uibDatepickerPopup`

Closes #4635
Fixes #4616

Author: Foxandxss

src/datepicker/datepicker.js

@@ -550,7 +550,7 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
       dateFormat = datepickerPopupConfig.html5Types[attrs.type];
       isHtml5DateInput = true;
     } else {
-      dateFormat = attrs.datepickerPopup || datepickerPopupConfig.datepickerPopup;
+      dateFormat = attrs.datepickerPopup || attrs.uibDatepickerPopup || datepickerPopupConfig.datepickerPopup;
       attrs.$observe('uibDatepickerPopup', function(value, oldValue) {
           var newDateFormat = value || datepickerPopupConfig.datepickerPopup;
           // Invalidate the $modelValue to ensure that formatters re-run