#### 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(handleUserResponse)
	.catch(handleError);
axios.put<User>('/user', { name: 'foo', id: 1 })
	.then(handleUserResponse)
	.catch(handleError);
axios.patch<User>('/user', { name: 'foo', id: 1 })
	.then(handleUserResponse)
  .catch(handleError);
const handleStringResponse = (response: string) => {
  console.log(response);
};
axios.get<User, string>('/user?id=12345')
  .then(handleStringResponse)
  .catch(handleError);
axios.get<User, string>('/user', { params: { id: 12345 } })
  .then(handleStringResponse)
  .catch(handleError);
axios.head<User, string>('/user')
  .then(handleStringResponse)
  .catch(handleError);
axios.options<User, string>('/user')
  .then(handleStringResponse)
  .catch(handleError);
axios.delete<User, string>('/user')
  .then(handleStringResponse)
  .catch(handleError);
axios.post<Partial<UserCreationDef>, string>('/user', { name: 'foo' })
  .then(handleStringResponse)
  .catch(handleError);
axios.post<Partial<UserCreationDef>, string>('/user', { name: 'foo' }, { headers: { 'X-FOO': 'bar' } })