Update UniquenessKey for when Element is/isn't nullable (#896)

[zannen]

With a schema:

type Query {
  things1: [Thing] # Note the lack of "!"
}

type Subscription {
  things2: [Thing!] # Note the "!"
}

the UniquenessKey for the two lists is the same, which causes non-deterministic output.

I have:

• Added tests covering the bug (see testing) - Note I realise I have not added tests. I took a look, but as the testing doc says, it's not easy. And this Issue causes flaky output, so I'm not sure what kind of test to write. Assistance requested.
  • Edit 1: In place of a unit test, I have added a check (https://github.com/zannen/gqlgen/commit/67f8031c02aef393b134f52ce0600d75b8aa20af) for obviously different TypeReferences being overwritten in the map.
  • Edit 2: I have added an example (https://github.com/zannen/gqlgen/commit/1a4230c8655f7f8ba66ffec86bce19b5a834826e) to check for regression. I think this is sufficient to check the box.
• Updated any relevant documentation (see docs) - Note I read all the docs, didn't see anywhere relevant to note this change.

[coveralls]

Coverage increased (+0.09%) to 62.162% when pulling 8a8f0a0 on zannen:master into 99a55da on 99designs:master.

[zannen]

I have no idea why the plugin/resolvergen race test is failing on CircleCI.

I ran the test over 2000 times locally and it succeeded every time:

while true ; do
  date
  go test -count=1 -race github.com/99designs/gqlgen/plugin/resolvergen
done

[vektah]

Nice one!