Fixed supported scopes handling when grant_type is client_credentials

When a storage was not returning an array, `$stored` was initialized with an empty array and an empty scope. The result of that is that when you tried to get an access token with a scope, even if it was set in the `supported_scopes` config variable, the server was always returning a `An unsupported scope was requested.` error.

With this PR, `$scope` is now initialized with `supported_scopes` config variable as default scope.