Merge branch 'file-message'

Author: stevebauman

src/Message.php

@@ -33,6 +33,14 @@ public function __sleep(): array
         return ['folder', 'uid', 'flags', 'headers', 'contents'];
     }
 
+    /**
+     * Get the message's folder.
+     */
+    public function folder(): Folder
+    {
+        return $this->folder;
+    }
+
     /**
      * Get the message's identifier.
      */