code: function graphiqlKoa(options) {
    return ctx => {
      const query = ctx.request.query;
      return resolveGraphiQLString(query, options, ctx)
        .then(graphiqlString => {
          ctx.set('Content-Type', 'text/html');
          ctx.body = graphiqlString;
        });
    };
  }
docs: This function is directly copied from:
  https://github.com/apollographql/apollo-server/blob/300c0cd12b56be439b206d55131e1b93a9e6dade/packages/apollo-server-koa/src/koaApollo.ts#L51
  
  And now this has been removed since v2 at:
  https://github.com/apollographql/apollo-server/commit/dbaa465646b0acb839860a85bfd68fb4379d64ab#diff-64af8fdf76996fa3ed4e498d44124800
  
  So we must keep this here to be compatible with what it was before.
  Thus users can directly use that by upgrading graphql to the v2 WITHOUT
  doing any other big changes.
  
  @param {Object} options The `options` of graphiqlKoa.
  @return {Promise} The result of the graphiqlKoa.