MockHttpServletRequest's getParameter(Values) returns null for null parameter name

Issue: SPR-10192
MockHttpServletResponse's getHeaderNames declares Collection instead of Set for Servlet 3.0 compatibility

Issue: SPR-9885
EntityManagerFactoryUtils finds default EntityManagerFactory in parent contexts as well

Also introduces consistent use of getBean(Class) for similar use cases across the framework, accepting a locally unique target bean even if further matching beans would be available in parent contexts (in contrast to BeanFactoryUtils.beanOfType's behavior).

Issue: SPR-10160
Final preparations for 3.1.4