apollo-server-koa: Respond to OPTIONS

[nounder]

Solves #2253

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[apollo-cla]

@RGTK: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[abernix]

LGTM, thanks!

[abernix]

This has been included in 2.4.3. I plan on promoting it to the latest tag tomorrow, but you can try this right now using the next tag or by directly installing 2.4.3. e.g.:

npm install apollo-server-koa@2.4.3