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

bindings/java/src/test/java/org/github/tursodatabase/jdbc4/JDBC4ConnectionTest.java

@@ -68,13 +68,16 @@ class JDBC4ConnectionTest {
 
   @Test
   void calling_close_multiple_times_throws_no_exception() throws Exception {
+    assertFalse(connection.isClosed());
     connection.close();
+    assertTrue(connection.isClosed());
     connection.close();
   }
 
   @Test
   void calling_methods_on_closed_connection_should_throw_exception() throws Exception {
     connection.close();
+    assertTrue(connection.isClosed());
     assertThrows(
         SQLException.class,
         () ->