Ensure user has a unique username otherwise `user10` would match `user1`

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/requests/api/members_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 describe API::Members, api: true  do
   include ApiHelpers
 
-  let(:master) { create(:user) }
+  let(:master) { create(:user, username: 'master_user') }
   let(:developer) { create(:user) }
   let(:access_requester) { create(:user) }
   let(:stranger) { create(:user) }