This commit introduces integration between MockMvc and HtmlUnit, thus
simplifying end-to-end testing when using HTML-based views and enabling
developers to do the following.

 - Easily test HTML pages using tools such as HtmlUnit, WebDriver, & Geb
   without the need to deploy to a Servlet container

 - Test JavaScript within pages

 - Optionally test using mock services to speed up testing

 - Share logic between in-container, end-to-end tests and
   out-of-container integration tests

Issue: SPR-13158