Usage of `self` instead of `window` broken on React Native

Commit [2c0a54](https://github.com/form-data/form-data/commit/2c0a54532c22da345e85afd559a9c6a14ec8155b) introduced `self` instead of `window` but this breaks in React Native - `self is undefined`. Maybe a different `require` implementation or something...
`window` works (as of v2.1.0).
