FIX: Revert formatting of sync_sso groups to match API

[bgastelo]

After upgrading from 0.47.0. to 0.48.1, I noticed our sync_sso calls started failing with 500s from Discourse. Reviewing the Discourse logs showed that it expects groups to be a comma separated string, not an array. This line here.

This is also mentioned in the setup doc:

If the discourse connect overrides groups option is specified, Discourse will consider the comma separated list of groups passed in groups.

I am running an older version of discourse (2.8.13) but I do not believe that this API endpoint has changed in new versions.

The PR that introduced this change was trying to modify DiscourseApi::SingleSignOn#groups, but changed both the #parse and #parse_hash methods. I believe #parse_hash is used exclusively for calling the sync_sso from discourse. Looks like an accidental change.

I am also modifying the spec to check the groups option as well. So this behavior is not accidentally changed again.

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If I'm wrong and there was an API change, feel free to ignore this PR.

Thanks