Allow use of search parameter for merge requests

It's possible to use search parameter for merge-requests. The only change is to add to $resolver.

Author: velichkin

lib/Gitlab/Api/MergeRequests.php

@@ -67,7 +67,8 @@ public function all($project_id, array $parameters = [])
             ->setAllowedTypes('created_before', \DateTimeInterface::class)
             ->setNormalizer('created_before', $datetimeNormalizer)
         ;
-
+        $resolver->setDefined('search');
+        
         return $this->get($this->getProjectPath($project_id, 'merge_requests'), $resolver->resolve($parameters));
     }