fix Logger image->string conversion

src/Logger.php

@@ -74,7 +74,7 @@ public function log($level, $message, array $context = [])
  {
  // `Vips\Image` to string convert
  array_walk_recursive($context, function (&$value) {
- if ($value instanceof Vips\Image) {
+ if ($value instanceof Image) {
  $value = (string) $value;
  }
  });