Supports already parsed documents when merging types

[kamilkisiela]

It allows for doing this:

import userType from './user.graphql';

const postType = `
  type Post {
    title: String!
  }
`;

mergeTypes([
  userType,
  postType
]);

Reason of polyfill.js with Object.values: jestjs/jest#3687 (comment)

[Urigo]

@RichardLitt is the king!

[RichardLitt]

Uh, thanks @Urigo. Not really relevant to this thread, though!

@kamilkisiela We'll get to this as soon as we can. :)

[kamilkisiela]

@RichardLitt great!

[RodMachado]

@kamilkisiela
LGTM, great job!