Add date formatting to MockHttpServletRequest and MockHttpServletResponse .

I improved MockHttpServletRequest.java and MockHttpServletResponse.java by adding date formatting.

The FastHttpDateFormat.java class added in org.springframework.mock.web package is a utility class to generate Http dates, and this class is based on code in Apache Tomcat.

Well, I found this class is already included in spring-security-web project as org.springframework.security.web.savedrequest.FastHttpDateFormat.java.

But because of project dependency problem, I can't use FastHttpDateFormat.java in spring-security-web and add same class in different package.

So spring team may have to think about how to solve FastHttpDateFormat class duplication.

Thanks.