Fixed handling of array keys in the tests

Author: GrahamCampbell

tests/Dotenv/Repository/RepositoryTest.php

@@ -66,8 +66,7 @@ private function key()
     {
         $keyVal = $this->keyVal();
 
-        /** @var string */
-        return \key($keyVal);
+        return (string) \key($keyVal);
     }
 
     /**