#### Is your feature request related to a problem? Please describe.
I want to use new toFormData method, but it is not typed in index.d.ts

#### Describe the solution you'd like
Add types for toFormData

#### Describe alternatives you've considered
Use ts-ignore

#### Additional context
None

test/typescript/axios.ts
axios.interceptors.response.eject(responseInterceptorId);
axios.interceptors.response.use(
  (response: AxiosResponse) => Promise.resolve(response),
  (error: any) => Promise.reject(error)
);
axios.interceptors.response.use((response: AxiosResponse) => response);
axios.interceptors.response.use((response: AxiosResponse) => Promise.resolve(response));
const adapter: AxiosAdapter = (config: AxiosRequestConfig) => {
  const response: AxiosResponse = {
    data: { foo: 'bar' },
    status: 200,
    statusText: 'OK',
    headers: { 'X-FOO': 'bar' },
    config
  };
  return Promise.resolve(response);
};
axios.defaults.adapter = adapter;
const promises = [
  Promise.resolve(1),
  Promise.resolve(2)
];
const promise: Promise<number[]> = axios.all(promises);
const fn1 = (a: number, b: number, c: number) => `${a}-${b}-${c}`;
const fn2: (arr: number[]) => string = axios.spread(fn1);
axios.get('/user')
  .then((response: AxiosResponse) => 'foo')
  .then((value: string) => {});
axios.get('/user')
  .then((response: AxiosResponse) => Promise.resolve('foo'))