Typings problem with PromiseStateMapping/FunctionMapping

Hello! Thanks to those who contributed the TypeScript definitions, they're very useful. I did run into what looks like a problem with the `PromiseStateMapping` and `FunctionMapping` [definitions](https://github.com/heroku/react-refetch/blob/7951bbf0e87cb1b71a91e89c69c5da0f3603028f/src/index.d.ts#L77). They look like:

```typescript
type PromiseStateMapping<
TProps,
TProp extends keyof TProps
> = TProps[TProp] extends PromiseState
? string | Mapping
: never;

// Function
type FunctionMapping<
TProps,
TProp extends keyof TProps
> = TProps[TProp] extends ((...args: infer TArgs) => void)
? ((...args: TArgs) => PropsMap)
: never;
```
The two types

```typescript
TProps[TProp] extends PromiseState
```

and

```typescript
TProps[TProp] extends ((...args: infer TArgs) => void)
```
appear to need an `| undefined` appended to them. Without this, I get (at least with TypeScript 3.3.3) type errors with `connect()` whenever the refetch props of the component are declared as optional with `?`. But all refetch props need to be declared optional, since otherwise users of the component would have to pass them in. Does this sound right or am I missing something?