Make SocketUtils a concrete class

Per the Javadoc for the SocketUtils() constructor, SocketUtils can be
instantiated as a Spring Bean in XML configuration files; however,
SocketUtils is currently abstract which prevents such usage.

This commit removes the 'abstract' declaration thereby allowing
SocketUtils to be instantiated as a Spring bean.

Issue: SPR-12169
(cherry picked from commit 559e81b)
Accept empty Content-Type header

Issue: SPR-12173
(cherry picked from commit 70412a9)
Client request implementations enforce RFC 6265 (cookies in a single header)

Issue: SPR-12196
(cherry picked from commit 26a93b6)
GenericTypeAwarePropertyDescriptor implements equals/hashCode for proper lookups on IBM JVM 6

Issue: SPR-12185
Polishing
Polishing
Latest dependency updates (Reactor 1.1.4)
Log warning for single optional constructor when no default constructor to fall back to

Issue: SPR-12161
(cherry picked from commit 80cec01)
BeanDefinitionParserDelegate does not silently ignore 1.x 'singleton' attribute

Issue: SPR-12167
(cherry picked from commit 0cf472b)
Polishing
ScheduledAnnotationBeanPostProcessor avoids needless re-scanning of non-annotated classes

Issue: SPR-12189
(cherry picked from commit 58b22ce)
Polishing
(cherry picked from commit a833889)
Explicit exclusion of bridge methods in annotation post-processors (for Java 8 compatibility)

Issue: SPR-12187
(cherry picked from commit f4219ca)
Backported tests for property source ordering

Issue: SPR-12198
(cherry picked from commit 90b93ff)
Polishing
(cherry picked from commit 16325c2)