Spring Simplify if-else constructions in spring-web

Removed redundant else-s in ReadOnlyHttpHeaders.java and in ResponseEntity.java

Comment From: sbrannen

Thanks for the PR.

For these particular scenarios, the inclusion of the else block is technically unnecessary; however, the original author introduced them for a reason, typically based on readability.

Since we do not have a project policy for "unnecessary else blocks", we tend to leave them in place.

In light of that, I am closing this PR.