SHINDIG-1958: Fix OAuth2TokenHandler#listToString()

1. Use Joiner instead of hand-rolling the joining.
2. Separate scopes by ' ' instead of ','

The first part actually makes the method "work" instead of throwing an AIOOBException, the second
part makes it "correct" according to the OAuth standard.

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



git-svn-id: https://svn.apache.org/repos/asf/shindig/trunk@1571417 13f79535-47bb-0310-9956-ffa450edef68