Before this change the getHttpEntityType method in
HttpEntityMethodProcessor raised an ISE if the generic type cannot be
detected. That made sense for resolving a controller method argument
where the target body type is crucial. However for a return value
the generic type should not be required since we either have an
actual body or no body at all in which case it doesn't even matter.
This change relaxes the checks and defaults to Object.class for the
ResponseEntity generic type on the return value side.
Issue: SPR-14799
71201e1a43