Google OAuth authentication does not deduplicate scopes

[jroper]

Multiple Google Alpakka libraries may require the same OAuth scope, and append the same scope to the list of scopes in their reference.conf file. If a user then includes both of those libraries in their project, then OAuth requests to Google will contain duplicate scopes, as far as I can tell from reading the code. While I don't think this will actually cause an issue, it's best that the scopes be deduplicated before sending them.