Support Access-Control-Allow-Credentials header

[praneshr]

Provide a way to define credentials: 'include' to the isomorphic-fetch.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials

[paulftw]

Since Dgraph servers don't use cookies and apps like Ratel are able to run queries and mutations without the credentials: include I personally don't see why this flag would be necessary.

Closing the issue as stale. Please re-open if there's a use case where setting the credentials thing is necessary.