Use `Object.fromEntries` to convert `Headers` into a plain object.
Use `Object.fromEntries` to convert `Headers` into a plain object.

As helpfully debugged and identified by @dsanders11 (Thank you!), this was a
mistake in my original implementation that incorrectly spread the iterable
`Headers` into the `headers` object of the response.

While I would have loved to create a regression test for this, there was no
place which was currently wired up to support handler specific options (like
`cors`), which is sometimes on the top-level constructor to `ApolloServer`,
and sometimes on `createHandler` or `applyMiddleware`.  I did try to futz
with it for a full hour to no avail.  I'm already aware of a number of
limitations with the current so-called `integration-testsuite`, and I'm
putting that on the list of things to address in an upcoming overhaul of
that testing infra.

That said, I've manually tested this and I think it's working properly.  An
upcoming release should hopefully confirm that.

Ref: https://github.com/apollographql/apollo-server/pull/2674/files#r288053103
Ref: https://github.com/apollographql/apollo-server/issues/2718#issuecomment-496263142

apollostack/apollo-server