Field merging

[nbudin]

PR Checklist:

• I have run npm test locally and all tests are passing.
• I have added/updated tests for any new behavior.
• If this is a significant change, an issue has already been created where the problem / solution was discussed: Multiple references to the same field within the same object are not merged #572
• If this is a version update, have you updated the CHANGELOG.md?

PR Description

This merges multiple references to the same field together when that field has subfields. It fixes both the simple case given in #572 and also the more complicated case involving fragment spreads.

I'm having some trouble reaching 100% test coverage with this. Jest is telling me that line 577 of GraphQLToOpenAPIConverter.ts is uncovered, but from what I can tell, that line is actually being invoked in the tests I added (if it wasn't, my tests should fail). I'd love any pointers on how to fix this!

[nbudin]

Found a bug while manually testing this that my test cases hadn't covered - working on a fix, will update here when it's done.

[nbudin]

After some more attempts at this, I'm actually going to give up. I'm sorry about this. It looks like while my original test case works ok, the reverse of it doesn't, e.g.:

query fieldMergingWithSubfieldFragmentSpread($id: ID!) {
  person(id: $id) {
    id
    firstName
    lastName

    address {
      country
    }

    ...addressSubfields
  }
}

I'm going to attempt to work around this in a different way on my end.