Accept empty Content-Type header

Issue: SPR-12173
(cherry picked from commit 70412a9)
Client request implementations enforce RFC 6265 (cookies in a single header)

Issue: SPR-12196
(cherry picked from commit 26a93b6)
Polishing
Log warning for single optional constructor when no default constructor to fall back to

Issue: SPR-12161
(cherry picked from commit 80cec01)
ScheduledAnnotationBeanPostProcessor avoids needless re-scanning of non-annotated classes

Issue: SPR-12189
(cherry picked from commit 58b22ce)
Polishing
(cherry picked from commit 50e50d0)