Fix/return string value (#58)

* fix return value
---------

Co-authored-by: korotkov <[email protected]>

Author: Miteugene

src/Converters/MixedValueToScalarConverter.php

@@ -15,7 +15,7 @@ public function handle(mixed $value): int|float|bool|string|null
         } elseif (is_object($value) && PHP_VERSION_ID >= 80100 && enum_exists(get_class($value))) {
             return $value->value;
         } elseif (is_object($value) && method_exists($value, '__toString')) {
-            $value->__toString();
+            return $value->__toString();
         }
 
         throw new BulkValueTypeIsNotSupported($value);