resolvers parameter should be optional for mergeSchemas

[kbrandwijk]

Per the documentation and runtime behavior. See #461

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• 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
• Update CHANGELOG.md with your change

[apollo-cla]

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[kbrandwijk]

There might be an added test that doesn't use any resolvers. As the tests are written in Typescript, that would immediately demonstrate that it's working. But the tests file for testMergeSchemas.ts is a bit hard to extend, because of the loops and the nesting.

[freiksenet]

Thanks!