SHINDIG-1958 [2/1]: Add missing unit test for OAuth2TokenHandler#listToString()

Missed in the previous commit :/

Review: https://reviews.apache.org/r/15958/



git-svn-id: https://svn.apache.org/repos/asf/shindig/trunk@1571418 13f79535-47bb-0310-9956-ffa450edef68