Convert Buffer.byteLength to string before assigning to `Content-Le…

…ngth`. (#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]: #907
[1]: #927
[2]: #939
[3]: https://circleci.com/gh/apollographql/apollo-server/1587

packages/apollo-server-express/src/expressApollo.ts

@@ -44,7 +44,10 @@ export function graphqlExpress(
     }).then(
       gqlResponse => {
         res.setHeader('Content-Type', 'application/json');
-        res.setHeader('Content-Length', Buffer.byteLength(gqlResponse, 'utf8'));
+        res.setHeader(
+          'Content-Length',
+          Buffer.byteLength(gqlResponse, 'utf8').toString(),
+        );
         res.write(gqlResponse);
         res.end();
       },