Spring DefaultServerHttpRequestBuilder can create case-sensitive Headers instances

In 6.2.0-M5, the DefaultServerHttpRequestBuilder was modified to ensure copying read-only headers would result in a mutable HttpHeaders instance. Unfortunately, the way this is implemented has a side effect: the case-insensitive nature of the headers is lost.

See https://github.com/spring-projects/spring-framework/pull/32097/files#r1791264218