Enable asyncSupported flag in Spring Test MVC

Also remove Servlet 3 bridge classes no longer needed since Spring
Framework 4 compiles with the Servlet 3 API. That means applications
may need to run tests with the Servlet 3 API jar although
technically they don't have to run with that in production.

Author: rstoyanchev

spring-test-mvc/src/main/java/org/springframework/test/web/servlet/request/MockAsyncContext.java

@@ -653,6 +653,8 @@ public final MockHttpServletRequest buildRequest(ServletContext servletContext)
 			Assert.notNull(request, "Post-processor [" + postProcessor.getClass().getName() + "] returned null");
 		}
 
+		request.setAsyncSupported(true);
+
 		return request;
 	}