Replace false for undefined

[DenisCarriere]

Pull Request #16

When I was using tiletype I noticed the types are getting mixed up (string | boolean).

Causes errors when a type is meant to be string.

type Formats = 'jpg' | 'png' 
const format: Formats = tiletype.type(buffer)

or

const format: string = tiletype.type(buffer)

Switching the return results to undefined should not break any libraries that tested for unknown buffer types.

This would still be valid

const format = tiletype.type(buffer)
if (format) { console.log('format exists!') }

or

const format = tiletype.type(buffer) | 'png'

Ran the tiletype tests and everything looked good, the only minor changes was handling the tiletype.headers function with undefined.