Typings: add call signature compatible with original fetch

[Guria]

Useful for classes that accepts fetch in constructor like:

class MyAjax {
  constructor(private fetch: Window['fetch']) {}
  get(/* ... */) {
    return this.fetch(/* ... */)
  }
  /* ... */
}

then in tests:

import MyAjax from './ajax'
import fetch from 'jest-fetch-mock'

test('', () => {
  fetch.mockResponseOnce(JSON.stringify({ data: '12345' }))
  const ajax = new MyAjax(fetch)
  ajax.get('/foo')
})

[jefflau]

Hi Guria,

I do not use typescript currently so I will page in @eryshev who did the original typescript definitions.

[alexeyeryshev]

Hi,

LGTM; 👍