Only support headers on uncommitted response

CrossOriginOpenerPolicyRequestCycleListener attempts to write headers to
a response that is already committed. It contains logic to prevent
setting of headers when a response does not support them, but the
implementation for ServletWebResponse always returns true.

This commit modifies ServletWebResponse to check if the underlying
HttpServletResponse has already been committed.

wicket-core/src/main/java/org/apache/wicket/protocol/http/servlet/ServletWebResponse.java

@@ -91,7 +91,7 @@ public void setDateHeader(String name, Instant date)
 	@Override
 	public boolean isHeaderSupported()
 	{
-		return true;
+		return !httpServletResponse.isCommitted();
 	}
 
 	@Override