Before this change the PathExtensionContentNegotiationStrategy accessed
the ServletContext via request.getServletContext, which is Servlet 3
specific. To work around it, there is now a Servlet-specific sub-class
that accepts a ServletContext as a constructor argument.
The ContentNegotiationManagerFactoryBean is now ServletContextAware and
if it has a ServletContext it creates the Servlet-specific sub-class
of PathExtensionContentNegotiationStrategy.
The ContentNegotiationManagerFactoryBean is now also used in several
places internally -- MVC namespace, MVC Java config, and the
ContentNegotiatingViewResolver -- to reduce duplication.
Issue: SPR-9826
7b30ffd522