Fix combineDataProviders doesn't work when returned by an async function

[fzaninotto]

Problem

I'm using combineDataProvider to combine a REST and a GraphQL API. As the GraphQL API needs to be built, I'm injecting the data provider as follows:

export const App = () => {
  const [dataProvider, setDataProvider] = useState<DataProvider | undefined>(
    undefined
  );
  useEffect(() => {
    buildDataProvider().then((provider) => setDataProvider(() => provider));
  }, []);

  if (!dataProvider) {
    return <div>Loading</div>;
  }

  return (
    <Admin dataProvider={dataProvider}>
      ...
    </Admin>
  );
};

The buildDataProvider function is async:

export const buildDataProvider = async () => {
  const graphQLDataProvider = await graphQlDataProvider({
    clientOptions: { uri: "http://localhost:3002/" },
  });
  const restDataProvider = jsonServerDataProvider("http://localhost:3001");
  const dataProvider = combineDataProviders((resource) => {
    switch (resource) {
      case "Product":
      case "Category":
        return graphQLDataProvider;
      default:
        return restDataProvider;
    }
  });
  return dataProvider;
};

This fails because of the way combineDataProvider handles then() calls.