Add strong typing for 3.0 version

src/Serializer/NoSerialize.php

@@ -5,7 +5,7 @@ namespace WPDesk\Forms\Serializer;
 use WPDesk\Forms\Serializer;
 
 class NoSerialize implements Serializer {
-	public function serialize( $value ): string {
+	public function serialize( $value ) {
 		return $value;
 	}