Fix tests

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

lib/gitlab_net.rb

@@ -15,10 +15,11 @@ class GitlabNet
       action: cmd,
       ref: ref,
       project: project_name,
-      oldrev: oldrev,
-      newrev: newrev
     }
 
+    params.merge!(oldrev: oldrev) if oldrev
+    params.merge!(newrev: newrev) if newrev
+
     if actor =~ /\Akey\-\d+\Z/
       params.merge!(key_id: actor.gsub("key-", ""))
     elsif actor =~ /\Auser\-\d+\Z/