Missing override for `hasAsKey()` in `AnnotationIntrospectorPair`

[cowtowncoder]

(root cause of FasterXML/jackson-modules-java8#206)

One small but important omission from #2871 implementation is that AnnotationIntrospectorPair was not augmented to deal with chained introspectors. As a result, no call will be made for either primary or secondary, when using pair...