feat(datepicker): add allowInvalid support

- Adds support for invalid view values via `ngModelOptions`
  allowInvalid` setting

Closes #4694
Closes #4837

Author: Jeffrey Barrus

Diff for: src/datepicker/datepicker.js

@@ -826,14 +826,12 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
 
     if (angular.isString(viewValue)) {
       var date = parseDateString(viewValue);
-      if (isNaN(date)) {
-        return undefined;
+      if (!isNaN(date)) {
+        return date;
       }
-
-      return date;
     }
 
-    return undefined;
+    return ngModel.$options && ngModel.$options.allowInvalid ? viewValue : undefined;
   }
 
   function validator(modelValue, viewValue) {

Diff for: src/datepicker/docs/readme.md

@@ -94,6 +94,11 @@ The datepicker has 3 modes:
 * `year-range`
   _(Default: `20`)_ -
   Number of years displayed in year selection.
+  
+* `ng-model-options`
+  _(Default: {})_ -
+  allowInvalid support. [More on ngModelOptions](https://docs.angularjs.org/api/ng/directive/ngModelOptions).
+
 
 ### uib-datepicker-popup settings ###
 

Diff for: src/datepicker/test/datepicker.spec.js

@@ -1266,6 +1266,32 @@ describe('datepicker', function() {
       });
     });
 
+    describe('ngModelOptions allowInvalid', function() {
+      var $sniffer, inputEl;
+
+      beforeEach(inject(function(_$sniffer_) {
+        $sniffer = _$sniffer_;
+
+        $rootScope.date = new Date('September 30, 2010 15:30:00');
+        $rootScope.modelOptions = {allowInvalid: true};
+        element = $compile('<div><input ng-model="date" ng-model-options="modelOptions" uib-datepicker-popup></div>')($rootScope);
+        inputEl = element.find('input');
+        $rootScope.$digest();
+      }));
+
+      function changeInputValueTo(el, value) {
+        el.val(value);
+        el.trigger($sniffer.hasEvent('input') ? 'input' : 'change');
+        $rootScope.$digest();
+      }
+
+      it('should update ng-model even if the date is invalid when allowInvalid is true', function() {
+        changeInputValueTo(inputEl, 'pizza');
+        expect($rootScope.date).toBe('pizza');
+        expect(inputEl.val()).toBe('pizza');
+      });
+    });
+
     describe('setting datepickerPopupConfig', function() {
       var originalConfig = {};
       beforeEach(inject(function(uibDatepickerPopupConfig) {