pubSub.asyncIterator() return type

Shouldn't the return type be `AsyncIterableIterator` ([TS lib file](https://github.com/Microsoft/TypeScript/blob/master/lib/lib.es2018.asynciterable.d.ts#L42)) instead of `AsyncIterator`? I see it has the `Symbol.asyncIterator` property [here](https://github.com/apollographql/graphql-subscriptions/blob/master/src/event-emitter-to-async-iterator.ts#L71)

Start with src/event-emitter-to-async-iterator.ts, especially the implementation around the Symbol.asyncIterator property, and inspect the declared return type of pubSub.asyncIterator(). Compare it with TypeScript's AsyncIterableIterator definition in lib.es2018.asynciterable.d.ts. Done means the public return type accurately reflects the implemented iterator.

Written by the indexing model from the issue text.