[#204] Fix Mongo OAuth2 adapter submission

- If no DSN is provided (i.e., the default will be used), the
  AuthenticationModel was not receiving a DSN in the payload, and thus not
  creating an AuthenticationEntity. The model now checks for "dsn_type" in the
  payload, which is guaranteed to be present for all OAuth2 types.

- Additionally, if no DSN is present, we provide a sane default.
Merge branch 'hotfix/204'

Close #204