client_credentials grant incorrectly requires scope

[sbxalex]

Requesting a token with grant_type=client_credentials results in a 400 Bad Request error that says {"error":"invalid_request","error_description":"mandatory: scope"}. This goes against the spec – see the relevant section, where it says scope is OPTIONAL (which, in turn, means implementations with such an OPTIONAL feature must be able to interoperate with those that don't, according to RFC 2119).