Convert `Buffer.byteLength` to string before assigning to `Content-Length`. (#956)
Convert `Buffer.byteLength` to string before assigning to `Content-Length`. (#956)

This change seems necessary in order to meet the new type definitions for
`res.setHeader` which mandate that the argument be a string, or an array of
strings.  Those type definitions were introduced via the `@types/node@9`
series of typing updates provided in [0], [1], [2].

With any luck, this will fix the failures being exhibited in CircleCI
tests[3] after we landed those typing updates.

[0]: https://github.com/apollographql/apollo-server/pull/907
[1]: https://github.com/apollographql/apollo-server/pull/927
[2]: https://github.com/apollographql/apollo-server/pull/939
[3]: https://circleci.com/gh/apollographql/apollo-server/1587
apollostack/apollo-server