set content type to text/html for playground

[jamiehodge]

BUGFIX: the playground page is currently being served without a content type

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]

@jamiehodge: 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/

[jamiehodge]

There doesn't appear to be an easy way to get the headers from the rp response as-is. Not sure if it's worth it.

[jamiehodge]

@abernix it would be great to get this small change in. The playground currently fails to render as HTML in some environments.

[abernix]

Thanks. No promises I'll get this out in a new release today, but I'll try.