Usually this request attribute is set for all sub-classes of
AbstractUrlHandlerMapping and therefore whenever
AnnotationMethodHandlerAdapter is used. However, having a
default value to fall back on in AnnotationMethodHandlerAdapter
is still appropriate in general and also considering the Javadoc
of HandlerMapping.INTROSPECT_TYPE_LEVEL_MAPPING.
Issue: SPR-9629
Backport-Issue: SPR-9633