Exposed all of XStream 1.4's configurable strategies as XStreamMarshaller bean properties

Issue: SPR-10421
Introduced "mapperWrappers" bean property

Issue: SPR-10421
Added javadoc note on the use of CachingConnectionFactory

Issue: SPR-10581
Fixed accidental use of JDK 8 getOrDefault method on MultiValueMap

Issue: SPR-10807
Polishing

Issue: SPR-10807
Extract ContentType as a base class of MediaType

ContentType is available in core-spring and does not include support
for quality parameters and media used in HTTP content negotiation.
The MediaType sub-class in org.springframework.http adds q-parameters.