simplify/robustify WebSocketHttpHeaders.java getSecWebSocketProtocol() ?

[jonenst]

Hi, while browsing the code, I was suprised by the logic in https://github.com/spring-projects/spring-framework/blob/main/spring-websocket/src/main/java/org/springframework/web/socket/WebSocketHttpHeaders.java#L162

	public List<String> getSecWebSocketProtocol() {
		List<String> values = get(SEC_WEBSOCKET_PROTOCOL);
		if (CollectionUtils.isEmpty(values)) {
			return Collections.emptyList();
		}
		else if (values.size() == 1) {
			return getValuesAsList(SEC_WEBSOCKET_PROTOCOL);
		}
		else {
			return values;
		}
	}

it would make the code "fail" when request comes with
protocol: foo
protocol: bar, baz

return ["foo", "bar, baz"] instead of ["foo", "bar", "baz"]

Also unify the quote handling (even though the rfc explicitly forbids quotes for sec-websocket-protocol, but at least this would be uniform. but even better would maybe be to reject or just ignore quotes ?) :
protocol: "foo", "bar" returns ["foo", "bar"]
vs
protocol: "foo"
protocol: "bar"
returns [""foo"", ""bar""]

It looks like it's the only place in the whole codebase with this extra check for size() == 1

$ git grep -C 3 "size() == 1" | grep -i AsList
spring-websocket/src/main/java/org/springframework/web/socket/WebSocketHttpHeaders.java-			return getValuesAsList(SEC_WEBSOCKET_PROTOCOL);

seems like it was iterated on in 55dae61#diff-e2d6218e6585f8b7e32682f6f8f7aed22dbd76d862ec92e86e0902243889556aL462-R471 and

It's obviously an edge case, but unless I missed somthing, it becomes simpler/more robust to just change it to

	public List<String> getSecWebSocketProtocol() {
			return getValuesAsList(SEC_WEBSOCKET_PROTOCOL);
	}

like in all methods in the parent class HttpHeaders.java (e.g. https://github.com/spring-projects/spring-framework/blob/main/spring-web/src/main/java/org/springframework/http/HttpHeaders.java#L706

	public List<String> getAccessControlAllowHeaders() {
		return getValuesAsList(ACCESS_CONTROL_ALLOW_HEADERS);
	}

I just saw this in passing, just though it'd mention it (obviously I'm leaving all the hard work of knowing what to do to you...). Feel free to close if appropriate