Convert Buffer.byteLength to string before assigning to Content-Length.

[abernix]

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.

[abernix]

All of the tests failed on the first run due to a separate typing failure in @types/koa due to its use of http2 which isn't available in Node 4, 6 or 8 (on 8 it is available but only if --expose-http2 is passed to node, which I don't believe we're doing).

First run: https://circleci.com/workflow-run/318777a7-cddf-4a79-90d9-a92a5da701a4

Subsequent run: https://circleci.com/workflow-run/a357627a-d90b-42cc-9b8b-6a6a09547d2b

I'm a bit perplexed as I fully expected the http2 usage to continue to fail until the types were rectified (or rolled back), not suddenly start working. The change in the types was introduced in DefinitelyTyped/DefinitelyTyped#24651, which should have surfaced as @types/koa@2.0.45 via #951.

[abernix]

Worth noting: It also failed in the Netlify build, so it wasn't isolated to CircleCI.

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Update: The "retry" of the Netlify build worked fine too. 🤷‍♂️