Fix the logic on determining whether connection is closed

bindings/java/src/main/java/org/github/tursodatabase/core/LimboConnection.java

@@ -31,7 +31,6 @@ public abstract class LimboConnection implements Connection {
   public LimboConnection(String url, String filePath, Properties properties) throws SQLException {
     this.database = open(url, filePath, properties);
     this.connectionPtr = this.database.connect();
-    this.closed = true;
   }
 
   private static AbstractDB open(String url, String filePath, Properties properties)

bindings/java/src/main/java/org/github/tursodatabase/jdbc4/JDBC4Connection.java

@@ -88,8 +88,7 @@ public class JDBC4Connection extends LimboConnection {
 
   @Override
   public boolean isClosed() throws SQLException {
-    // TODO
-    return false;
+    return super.isClosed();
   }
 
   @Override