Remove unnecessary null check

Issue: SPR-12378
Merge pull request #677 from ksseo0701/SPR-12378

* SPR-12378:
  Remove unnecessary null check
Corrected note on JPA <exclude-unlisted-classes>

Issue: SPR-10767
Test for <util:map> with LinkedCaseInsensitiveMap and specified key/value types

Issue: SPR-10994
Fix a package tangle between SockJS support and transport packages

Issue: SPR-12379
Replace "if(" with "if ("
TransactionAspectSupport.currentTransactionStatus() reliably throws NoTransactionException

Issue: SPR-9144
PropertyValue declares its cache fields as transient

Issue: SPR-12377
Netty support for (Async)RestTemplate

This commit introduces an AsyncClientHttpRequestFactory based on Netty
4, for use with the (Async)RestTemplate.
Add Netty version property to build.gradle

Issue: SPR-12381
Polish netty client support

Mostly updates based on:
https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Code-Style
Fixed isWritable for badly formed SpEL expressions

Before this change isWritable() could return true
for a badly formed expression. That is because the
decision about whether something is writable was made
based on the node type rather than whether the node
represented something that could actually be resolved
to be a real thing. This change ensures a resolution
check is done and isWritable() should only return
true if a subsequent setValue() will succeed.

Issue: SPR-10610
Allow HTTP delete with request entity

Issue: SPR-12361
Minor doc update

Issue: SPR-12375