ContentNegotiationManagerFactoryBean now ensures that
ContentNegotiationManager contains the MediaType mappings even if the
path extension and the parameter strategies are off.
There are also minor fixes to ensure the media type mappings in
ContentNegotiationManagerFactoryBean aren't polluted when mapping keys
are not lowercase, and likewise MappingMediaTypeFileExtensionResolver
filters out duplicates in the list of all file extensions.
See gh-24179
542e187831