use resolvers for unions and interface type in mockServer

[itajaja]

mockServer ignores the resolve function for implementation of union or interface types. For example this mockMap:

const mockMap = {
      Int: () => 12345,
      Bird: () => ({ returnInt: () => 54321 }),
      Bee: () => ({ returnInt: () => 54321 }),
    };

on this schema:

type Bird {
  returnInt: Int
}

type Bee {
  returnInt: Int
}

union BirdsAndBees = Bird | Bee

type RootQuery {
  returnBirdsAndBees: [BirdsAndBees]
}

schema {
  query: RootQuery
}

with the following query:

  {
      returnBirdsAndBees {
        ... on Bird {
          returnInt
          returnString
        }
        ... on Bee {
          returnInt
          returnEnum
        }
      }
    }

returns 12345 instead of 54321