Don't add fields to the root mutation/query

[amandajliu]

TODO:

• Update CHANGELOG.md with your change
• 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

[rricard]

@stubailo: this PR introduces a change that breaks the apolloFetchMore directive stripping. The way it has been done suppresses all query transformers on root query.

[rricard]

See #374