Spring UndertowHeadersAdapter's remove() method violates Map contract

The remove(Object key) method in UndertowHeadersAdapter should return any previous value stored under the key; however, the current implementation always returns null.

This has already been fixed in main as part of commit https://github.com/spring-projects/spring-framework/commit/785212d67655119a71d3ad700e90c6e09e7c6361#diff-8be9fc708f521a68eb27761d98ddd6ebc62ec5431095673c7273647a538fae6bR132-R136.