title: Angular4 "required" attribute is not checked on form submit
body: <p>My very basic Angular 4 form doesn't validate the input (text) fields with the "required" attribute. Input with "required" attribute should be validated by the browser like the html 5 default behaviour without Angular. Was there any change in Angular 4? The same code worked fine with Angular 2.</p>
  
  <pre><code>&lt;form (ngSubmit)="onSubmit()" #testForm="ngForm"&gt;
    &lt;input type="text" id="test1" name="test1" [(ngModel)]="test" required&gt;
    &lt;input type="text" id="test2" name="test2" [(ngModel)]="test" [required]="true"&gt;
    &lt;input type="text" id="test3" name="test3" [(ngModel)]="test" [attr.required]="true"&gt;
    &lt;button type="submit"&gt;submit&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>Try in this plunkr:
  <a href="http://plnkr.co/edit/Wow7NUhaC0KjHtOfVQ2c?p=preview" rel="nofollow noreferrer">http://plnkr.co/edit/Wow7NUhaC0KjHtOfVQ2c?p=preview</a></p>