Support for java.util.Optional within ObjectFactory/Provider

Includes support for arbitrary deep nesting levels in DependencyDescriptor's getDependencyType() and MethodParameter's getNestedParameterType().

Issue: SPR-11833
Update Javadoc on SockJS client library URL

Issue: SPR-11766
Improve no content handling in MockHttpServletRequest

Issue: SPR-11764
Fix issue with RequestBody(required=true)

Issue: SPR-11828
AbstractAutowireCapableBeanFactory's getTypeForFactoryBean considers FactoryBean<Object> declarations as non-indicative (just like raw declarations)

Issue: SPR-11842
Refined tests for FactoryBean return type resolution on @Bean methods

Issue: SPR-11842
Polishing
Reference documentation consistently refers to Apache HttpComponents instead of outdated Commons HttpClient

Includes overhaul of remaining references to J2SE and J2EE, consistently replacing them with "the JDK" and "Java EE".

Issue: SPR-11853