Allow custom ServletRequestAttributes handling

Add ServletRequestAttributes.getSubsequentRequestAttributes(...) method
that can be used to determine how subsequent HttpServletRequests should
be handled. The default implementation restores the behavior of Spring
3.1 in never replacing custom subclasses of ServletRequestAttributes.

The ServletTestExecutionListener from spring-test now override
getSubsequentRequestAttributes to ensure that the initial request
is alway replaced in the test context (see ).

Issue: