Fix type validations being to strict for dates (#6470)

* Fix type validations being to strict for dates

* Add changelog entry

changelog.md

+# Future
+- [FIXED] Accept dates as string while using `typeValidation` [#6453](https://github.com/sequelize/sequelize/issues/6453)
+
 # 4.0.0-1
 - [CHANGED] Removed `modelManager` parameter from `Model.init()` [#6437](https://github.com/sequelize/sequelize/issues/6437)
 - [FIXED] Made `Model.init()` behave like `sequelize.define()` (hooks are called and options have proper defaults) [#6437](https://github.com/sequelize/sequelize/issues/6437)

lib/data-types.js

@@ -429,7 +429,7 @@ DATE.prototype.toSql = function toSql() {
   return 'DATETIME';
 };
 DATE.prototype.validate = function validate(value) {
-  if (!_.isDate(value)) {
+  if (!Validator.isDate(String(value))) {
     throw new sequelizeErrors.ValidationError(util.format('%j is not a valid date', value));
   }
 

test/unit/model/validation.test.js

@@ -268,7 +268,8 @@ describe(Support.getTestDialectTeaser('InstanceValidator'), function() {
       name: Sequelize.STRING,
       awesome: Sequelize.BOOLEAN,
       number: Sequelize.DECIMAL,
-      uid: Sequelize.UUID
+      uid: Sequelize.UUID,
+      date: Sequelize.DATE
     });
 
     before(function () {
@@ -297,6 +298,14 @@ describe(Support.getTestDialectTeaser('InstanceValidator'), function() {
           })).not.to.be.rejected;
         });
 
+        it('should allow dates as a string', function() {
+          return expect(User.find({
+            where: {
+              date: '2000-12-16'
+            }
+          })).not.to.be.rejected;
+        });
+
         it('should allow decimal big numbers as a string', function () {
           return expect(User.create({
             number: '2321312301230128391820831289123012'