Improve multi-valued HTTP headers support

Prior to this change, getting header values with `HttpHeaders` when
headers are multi-valued would cause issues.
For example, for a given HTTP message with headers:
    Cache-Control: public, s-maxage=50
    Cache-Control: max-age=42

Getting a `List` of all values would return <"public", "s-maxage=50">
and getting the header value would return "public, s-maxage=50".

This commit takes now into account multi-valued HTTP headers and adds
new getters/setters for "If-Match" and "If-Unmodified-Since" headers.

Note that for ETag-related headers such as "If-Match" and
"If-None-Match", a special parser has been implemented since ETag values
can contain separator characters.

Issue: SPR-14223, SPR-14228

spring-websocket/src/main/java/org/springframework/web/socket/WebSocketHttpHeaders.java

@@ -172,7 +172,7 @@ public class WebSocketHttpHeaders extends HttpHeaders {
 			return Collections.emptyList();
 		}
 		else if (values.size() == 1) {
-			return getFirstValueAsList(SEC_WEBSOCKET_PROTOCOL);
+			return getValuesAsList(SEC_WEBSOCKET_PROTOCOL);
 		}
 		else {
 			return values;