[YouTube] Make detection of age restricted pages more reliable

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeStreamExtractor.java

@@ -626,8 +626,7 @@ public class YoutubeStreamExtractor extends StreamExtractor {
 
         final String playerUrl;
         // Check if the video is age restricted
-        if (pageContent.contains("<meta property=\"og:restrictions:age")) {
-            // do this if it is age gated
+        if (!doc.select("meta[property=\"og:restrictions:age\"").isEmpty()) {
             final EmbeddedInfo info = getEmbeddedInfo();
             final String videoInfoUrl = getVideoInfoUrl(getId(), info.sts);
             final String infoPageResponse = downloader.download(videoInfoUrl);