Fixed GraphQL Playground with custom configuration in production.

[nderscore]

This fixes issue #1386. Many thanks to @thinkjson for investigating and raising this issue, as well as the proposed solution.

This is an alternative to PR #1417. While that PR seeked to solve the issue by altering the playground configuration object to add a new option for enabling/disabling playground, this solution is more aligned with the existing configuration and (my) expectations about how it should behave.

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Fixed support for GraphQL Playground with custom configuration in production environments.

• Updated logic in createPlaygroundOptions to cast the playground option to a boolean
  if it is defined. Fall back to isDev logic only if undefined.
• Updated unit test for partial graphql playground options to emulate a 'production'
  environment, mimicking the scenario that was broken before.

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]

@nderscore: 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/

[nderscore]

CircleCI tests are currently failing due to issues unrelated to this PR. Will rebase this branch once it's fixed on master.

Edit: Tests seem to be passing now.

[evans]

@nderscore Thank you so much! We'll get this out asap