Fix #962, Enum internal value mapping

[tgriesser]

Fixes enum internal value mapping regression shown in #962, I believe this is also the issue with apollographql/apollo-server#1721, includes tests.

Basically it should be:

enumValueMap[type.name] = enumValueMap[type.name] || {};
enumValueMap[type.name][fieldName] = resolverValue[fieldName];

not

enumValueMap[type.name] = {
  [fieldName]: resolverValue[fieldName],
};

Unrelated, it'd be nice to get a .prettierrc and/or prettier setup as a precommit hook so it runs with the project's preferred settings.

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. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[hwillson]

Awesome - thanks @tgriesser!