added support for has method

src/Decorator/DelaySinglePersistentContainer.php

@@ -40,6 +40,15 @@ final class DelaySinglePersistentContainer implements DelayPersistentContainer {
 		throw new ElementNotExistsException( sprintf( 'Element %s not exists!', $id ) );
 	}
 
+	public function has( $id ) {
+		if ( isset( $this->changed[ $id ] ) && $this->changed[ $id ] ) {
+			return isset( $this->internal_data[ $id ] );
+		}
+
+		$data = \maybe_unserialize( $this->container->get( $this->key ) );
+		return is_array( $data ) && isset( $data[ $id ] );
+	}
+
 	public function save() {
 		if ( $this->is_changed() ) {
 			$this->container->set( $this->key, \maybe_serialize( $this->internal_data ) );