Fix: Serialize arrays as JSON on fetch in RESTDataSource

[aireyc1]

Fixes issue introduced in #1125 and logged in #2186 and #2196.

Arrays are valid JSON per spec: https://tools.ietf.org/html/rfc7159

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]

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[abernix]

It's obviously not common to sub-class an Array, but could/should we switch this to use Array.isArray instead to help accommodate that?

[aireyc1]

@abernix Works for me. Updated.

[abernix]

LGTM. Thanks!