#### 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
.catch(handleError);
instance1.post('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
instance1.post('/user', { foo: 'bar' }, { headers: { 'X-FOO': 'bar' } })
  .then(handleResponse)
  .catch(handleError);
axios.defaults.baseURL = 'https://api.example.com/';
axios.defaults.headers.common['Authorization'] = 'token';
axios.defaults.headers.post['X-FOO'] = 'bar';
axios.defaults.timeout = 2500;
instance1.defaults.baseURL = 'https://api.example.com/';
instance1.defaults.headers.common['Authorization'] = 'token';
instance1.defaults.headers.post['X-FOO'] = 'bar';
instance1.defaults.timeout = 2500;
const requestInterceptorId: number = axios.interceptors.request.use(
  (config: AxiosRequestConfig) => config,
  (error: any) => Promise.reject(error)
);
axios.interceptors.request.eject(requestInterceptorId);
axios.interceptors.request.use(
  (config: AxiosRequestConfig) => Promise.resolve(config),
  (error: any) => Promise.reject(error)
);
axios.interceptors.request.use((config: AxiosRequestConfig) => config);
axios.interceptors.request.use((config: AxiosRequestConfig) => Promise.resolve(config));
const responseInterceptorId: number = axios.interceptors.response.use(
  (response: AxiosResponse) => response,
  (error: any) => Promise.reject(error)
);