Apollo-link: Enable `credentials: 'same-origin'` by default (or allow an option to enable it easily)

The old ApolloClient automatically passed cookies.
HttpLink, BatchHttpLink uses apollo-fetch, which uses isomorphic-fetch. By default, isomorphic-fetch doesn't send cookies.

The current workaround is pretty complicated:

    const uri = '/api/graphql';

    // by default, apollo-link-http doesn't send cookies
    const customFetch = (uri, options) =>
        fetch(uri, {
            ...options,
            credentials: 'same-origin',
        });

    // create an apollo fetch instance with our hacked fetch instance
    const apolloFetch = createApolloFetch({
        uri,
        customFetch,
    });

    const link = new BatchHttpLink({ uri, fetch: apolloFetch })

This problem can be solved in multiple ways:

1) we could enable it by default to mimic the old version
2) we could add another option to apollo-link-http and apollo-link-batch-http, called fetchOption, so we could just do something like

const fetchOptions = {
    credentials: 'same-origin',
};

const link = new BatchHttpLink({ uri, fetchOptions });

I'm willing to do the fix + the tests, just need help on figuring which way we want to go :-)

Intended outcome:
Cookies should be sent when doing a request

Actual outcome:
No cookies are being sent