TypeScript compiler error with ref passed from getInputProps to component using a forwardRef

- `downshift` version: 3.2.10
- `node` version: v11.15.0
- `npm` (or `yarn`) version: 6.7.0
- `react` version: 16.8.4
- `styled-components` version: 4.3.1

**Relevant code or config**

```
import * as React from "react";
import { render } from "react-dom";
import styled from "styled-components";
import Downshift from "downshift";

const Input = styled.input`
width: 200px;
`;

function App() {
return (

{({ getInputProps }) => (

const rootElement = document.getElementById("root");
render(, rootElement);

```

**What you did**: Attempted to pass a custom `getInputProps` to a component which implements a forward ref (in this case, to an `` element).

**What happened**: A compiler error from TypeScript:
```
Type '{ onKeyUp: ((e: KeyboardEvent) => void) & ((event: KeyboardEvent) => void); disabled?: boolean; accept?: string; acceptCharset?: string; action?: string; ... 354 more ...; key?: Key; }' is not assignable to type 'Pick, HTMLInputElement>, "form" | "style" | "title" | "pattern" | "onChange" | "onSelect" | "children" | ... 277 more ... | "onTransitionEndCapture"> & { ...; }, "form" | ... 284 more ... | "onTransitionEndCapture"> & Partial<...>, "form" | ... 284 mo...'.
Types of property 'ref' are incompatible.
Type 'LegacyRef' is not assignable to type '((instance: HTMLInputElement) => void) | RefObject'.
Type 'string' is not assignable to type '((instance: HTMLInputElement) => void) | RefObject'.ts(2322)
```

**Reproduction repository**: https://codesandbox.io/s/loving-diffie-njech

**Problem description**: Type of `ref` is incompatible, even though the component appears to behave correctly with the forwarded ref.

**Suggested solution**: Change the type definitions to (optionally?) remove the `LegacyRef` part.