Add default ContentTypeResolver initialization

Previously AbstractMessageConverter did not have a ContentTypeResolver
configured by default. However the Java config and XML namespace in
spring-messaging and spring-websocket always configured one.

This change ensures every AbstractMessageConverter is configured with an
instance of DefaultContentTypeResolver by default. This makes sense since
all the resolver does is make an attempt to find a content type to use
for matching. If it can't it returns null and it's up to the converter
to decide whether it can convert or not.

Issue: SPR-11462
Remove ContentTypeResolver from composite converter

Before this change CompositeMessageConverter had a ContentTypeResolver
field that was in turn set on all contained converters.

After this change that field is removed and effectively
CompositeMessageConverter is a simple container of other converters.
Each converter in turn must have been configured with a
ContentTypeResolver.

Doing so means it is less likely to have unexpected consequences when
configuring converters, the ContentTypeResolver set in the composite
converter overriding the one configured in a contained converter.

Also commit 676ce6 added default ContentTypeResolver initialization
to AbstractMessageConverter, which ensures that converters are still
straight forward to configure.

Issue: SPR-11462