3.0.3 seems to have incorrect typings

[keeganwatkins]

The types include this definition:

// Truncated for brevity
export interface FetchMock {
  (input?: string | Request, init?: RequestInit): Promise<Response>;
}

but in the spec, input cannot be undefined.

In Chrome, fetch() (invoked without arguments) produces a TypeError:

Failed to execute 'fetch' on 'Window': 1 argument required, but only 0 present.

[keeganwatkins]

I took a pass at resolving this in #207, I've not contributed to this project before so please let me know what I can do to help 😄