#### 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
.then(handleStringResponse)
  .catch(handleError);
axios.put<Partial<UserCreationDef>, string>('/user', { name: 'foo' })
  .then(handleStringResponse)
  .catch(handleError);
axios.patch<Partial<UserCreationDef>, string>('/user', { name: 'foo' })
  .then(handleStringResponse)
  .catch(handleError);
axios.request<User, string>({
  method: 'get',
  url: '/user?id=12345'
})
  .then(handleStringResponse)
  .catch(handleError);
const instance1: AxiosInstance = axios.create();
const instance2: AxiosInstance = axios.create(config);
instance1(config)
  .then(handleResponse)
  .catch(handleError);
instance1.request(config)
  .then(handleResponse)
  .catch(handleError);
instance1.get('/user?id=12345')
  .then(handleResponse)
  .catch(handleError);
instance1.options('/user')
  .then(handleResponse)
  .catch(handleError);
instance1.get('/user', { params: { id: 12345 } })
  .then(handleResponse)