Unwrap non-null types

[jgwhite]

We ran into the following error due to having non-null edges fields in our schema:

type FooConnection {
  edges: [FooNode!]!
  #              ↑ ↑
}

Cannot read property 'cursor' of undefined

And a similar issue with non-null mutation return types:

mutation {
  createFoo(input: CreateFooInput!): CreateFooResult!
  #                                                 ↑
}

Cannot read property 'replace' of undefined

---

This PR adds examples of non-null edges and non-null mutation return types to the schema and the accompanying logic to unwrap non-null types as required.

We deliberately modified only one of the two occurrences of edges to ensure it continues to work both ways.

We’ve been slightly lazy an amended the single existing mutation rather than adding a second mutation to test against. Happy to add an explicit mutation example if needsbe.

[jgwhite]

Looks like CI is failing because the coverage report isn’t being generated:

cat: coverage/lcov.info: No such file or directory

[jneurock]

Hey! Thanks for the contribution. It looks like the coverage issues were an issue with Travis.

[jgwhite]

Awesome. Thanks @jneurock!