RestClientResponseException handles responseHeaders case-sensitive

[MnlK]

Starting with spring-web 6.1.2, the HttpHeaders supplied to RestClientResponseException are copied into a new LinkedMultiValueMap, that is not case-insensitive anymore.
Headers from getResponseHeaders() can then no longer be found without the correct case.

public RestClientResponseException(
		String message, HttpStatusCode statusCode, String statusText, @Nullable HttpHeaders headers,
		@Nullable byte[] responseBody, @Nullable Charset responseCharset) {
	...
	this.responseHeaders = copyHeaders(headers);
	...
}

private static HttpHeaders copyHeaders(@Nullable HttpHeaders headers) {
	if (headers != null) {
		MultiValueMap<String, String> result = new LinkedMultiValueMap<>(headers.size());
		headers.forEach((name, values) -> values.forEach(value -> result.add(name, value)));
		return HttpHeaders.readOnlyHttpHeaders(result);
	}
	else {
		return null;
	}
}

[bclozel]

See #31787