8274949: Use String.contains() instead of String.indexOf() in java.base

Reviewed-by: weijun, dfuchs, vtewari, lancea

src/java.base/share/classes/java/net/HttpURLConnection.java

@@ -600,7 +600,7 @@ public abstract class HttpURLConnection extends URLConnection {
     public long getHeaderFieldDate(String name, long Default) {
         String dateString = getHeaderField(name);
         try {
-            if (dateString.indexOf("GMT") == -1) {
+            if (!dateString.contains("GMT")) {
                 dateString = dateString+" GMT";
             }
             return Date.parse(dateString);